lnworker: use config lightning fee for estimate

diff --git a/electrum/lnutil.py b/electrum/lnutil.py
index 28f1f4e..0d30d37 100644
--- a/electrum/lnutil.py
+++ b/electrum/lnutil.py
@@ -2030,7 +2030,8 @@ class PaymentFeeBudget(NamedTuple):
     # cltv-delta the destination wants for itself. (e.g. "min_final_cltv_delta" is excluded)
     cltv: int  # this is cltv-delta-like, no absolute heights here!
 
-    #num_htlc: int
+    PAYMENT_FEE_CUTOFF_CLAMP = 10_000_000  # [0, 10k sat]
+    PAYMENT_FEE_MILLIONTHS_CLAMP = 250_000  # [0, 25%]
 
     @classmethod
     def from_invoice_amount(
@@ -2067,8 +2068,8 @@ class PaymentFeeBudget(NamedTuple):
             fee_millionths = config.LIGHTNING_PAYMENT_FEE_MAX_MILLIONTHS
         if fee_cutoff_msat is None:
             fee_cutoff_msat = config.LIGHTNING_PAYMENT_FEE_CUTOFF_MSAT
-        millionths_clamped = min(max(0, fee_millionths), 250_000)  # clamp into [0, 25%]
-        cutoff_clamped = min(max(0, fee_cutoff_msat), 10_000_000)  # clamp into [0, 10k sat]
+        millionths_clamped = min(max(0, fee_millionths), cls.PAYMENT_FEE_MILLIONTHS_CLAMP)
+        cutoff_clamped = min(max(0, fee_cutoff_msat), cls.PAYMENT_FEE_CUTOFF_CLAMP)
         if fee_millionths != millionths_clamped:
             _logger.warning(
                 f"PaymentFeeBudget. found insane fee millionths in config. "
@@ -2082,3 +2083,29 @@ class PaymentFeeBudget(NamedTuple):
         fee_msat = invoice_amount_msat * millionths_clamped // 1_000_000
         fee_msat = max(fee_msat, cutoff_clamped)
         return fee_msat
+
+    @classmethod
+    def reverse_from_total_amount(cls, *, total_amount_msat: int, config: 'SimpleConfig') -> int:
+        """
+        Given the total amount (including fees) return the amount of fees
+        included assuming highest allowed from config fees are being used.
+
+        This allows to guess a fee that has to be reserved to reliably allow
+        doing a "Max" amount lightning send (e.g. for submarine swaps).
+        """
+        assert isinstance(total_amount_msat, int) and total_amount_msat >= 0, repr(total_amount_msat)
+
+        millionths_clamped = min(
+            max(0, config.LIGHTNING_PAYMENT_FEE_MAX_MILLIONTHS),
+            cls.PAYMENT_FEE_MILLIONTHS_CLAMP,
+        )
+        cutoff_clamped = min(
+            max(0, config.LIGHTNING_PAYMENT_FEE_CUTOFF_MSAT),
+            cls.PAYMENT_FEE_CUTOFF_CLAMP,
+        )
+
+        # inverse of _calculate_fee_msat
+        amount_minus_fees = (total_amount_msat * 1_000_000) // (1_000_000 + millionths_clamped)
+        fees_msat = max(total_amount_msat - amount_minus_fees, cutoff_clamped)
+        fees_msat = min(fees_msat, total_amount_msat)  # to handle (invalid?) inputs below cutoff_clamped
+        return fees_msat
diff --git a/electrum/lnworker.py b/electrum/lnworker.py
index 347bcc0..d3074ab 100644
--- a/electrum/lnworker.py
+++ b/electrum/lnworker.py
@@ -22,6 +22,7 @@ from concurrent import futures
 import urllib.parse
 import itertools
 import dataclasses
+from math import ceil
 
 import aiohttp
 import dns.asyncresolver
@@ -2950,15 +2951,17 @@ class LNWallet(LNWorker):
                 if self.channel_db or self.is_trampoline_peer(c.node_id):
                     yield c
 
-    def fee_estimate(self, amount_sat):
-        # Here we have to guess a fee, because some callers (submarine swaps)
-        # use this method to initiate a payment, which would otherwise fail.
-        fee_base_msat = 5000               # FIXME ehh.. there ought to be a better way...
-        fee_proportional_millionths = 500  # FIXME
-        # inverse of fee_for_edge_msat
-        amount_msat = amount_sat * 1000
-        amount_minus_fees = (amount_msat - fee_base_msat) * 1_000_000 // ( 1_000_000 + fee_proportional_millionths)
-        return Decimal(amount_msat - amount_minus_fees) / 1000
+    def estimate_fee_reserve_for_total_amount(self, amount_sat: int | Decimal) -> int:
+        """
+        Estimate how much of the given amount needs to be reserved for
+        ln payment fees to reliably pay the remaining amount.
+        """
+        amount_msat = ceil(amount_sat * 1000)  # round up to the next sat
+        fee_msat = PaymentFeeBudget.reverse_from_total_amount(
+            total_amount_msat=amount_msat,
+            config=self.config,
+        )
+        return ceil(Decimal(fee_msat) / 1000)
 
     def num_sats_can_send(self, deltas=None) -> Decimal:
         """
@@ -2985,7 +2988,7 @@ class LNWallet(LNWorker):
                     can_send_dict[c.node_id] += send_capacity(c)
         can_send = max(can_send_dict.values()) if can_send_dict else 0
         can_send_sat = Decimal(can_send)/1000
-        can_send_sat -= self.fee_estimate(can_send_sat)
+        can_send_sat -= self.estimate_fee_reserve_for_total_amount(can_send_sat)
         return max(can_send_sat, 0)
 
     def get_channels_for_receiving(
@@ -3083,7 +3086,7 @@ class LNWallet(LNWorker):
             for chan in channels:
                 available_sat = chan.available_to_spend(LOCAL if direction == SENT else REMOTE) // 1000
                 delta = amount_sat - available_sat
-                delta += self.fee_estimate(amount_sat)
+                delta += self.estimate_fee_reserve_for_total_amount(amount_sat)
                 # add safety margin
                 delta += delta // 100 + 1
                 if func(deltas={chan:delta}) >= amount_sat:
@@ -3106,7 +3109,7 @@ class LNWallet(LNWorker):
             return False
         for chan2, delta in suggestions:
             # margin for fee caused by rebalancing
-            delta += self.fee_estimate(amount_sat)
+            delta += self.estimate_fee_reserve_for_total_amount(amount_sat)
             # find other channel or trampoline that can send delta
             for chan1 in self.channels.values():
                 if chan1.is_frozen_for_sending() or not chan1.is_active():
@@ -3129,7 +3132,7 @@ class LNWallet(LNWorker):
     def num_sats_can_rebalance(self, chan1, chan2):
         # TODO: we should be able to spend 'max', with variable fee
         n1 = chan1.available_to_spend(LOCAL)
-        n1 -= self.fee_estimate(n1)
+        n1 -= self.estimate_fee_reserve_for_total_amount(n1)
         n2 = chan2.available_to_spend(REMOTE)
         amount_sat = min(n1, n2) // 1000
         return amount_sat
diff --git a/tests/test_lnutil.py b/tests/test_lnutil.py
index 944d8a8..944fcc8 100644
--- a/tests/test_lnutil.py
+++ b/tests/test_lnutil.py
@@ -10,7 +10,7 @@ from electrum.lnutil import (
     derive_privkey, derive_pubkey, make_htlc_tx, extract_ctn_from_tx, get_compressed_pubkey_from_bech32,
     ScriptHtlc, calc_fees_for_commitment_tx, UpdateAddHtlc, LnFeatures, ln_compare_features,
     IncompatibleLightningFeatures, ChannelType, offered_htlc_trim_threshold_sat, received_htlc_trim_threshold_sat,
-    ImportedChannelBackupStorage, list_enabled_ln_feature_bits
+    ImportedChannelBackupStorage, list_enabled_ln_feature_bits, PaymentFeeBudget,
 )
 from electrum.util import bfh, MyEncoder
 from electrum.transaction import Transaction, PartialTransaction, Sighash
@@ -1121,3 +1121,33 @@ class TestLNUtil(ElectrumTestCase):
             ),
             decoded_cb,
         )
+
+    async def test_payment_fee_budget(self):
+        config = SimpleConfig()
+        # test value above cutoff
+        invoice_amount_msat = 1_000_000 * 1000
+        budget = PaymentFeeBudget.from_invoice_amount(
+            invoice_amount_msat=invoice_amount_msat,
+            config=config,
+        )
+        reversed_fee_msat = PaymentFeeBudget.reverse_from_total_amount(
+            total_amount_msat=invoice_amount_msat + budget.fee_msat,
+            config=config,
+        )
+        self.assertGreater(budget.fee_msat, config.LIGHTNING_PAYMENT_FEE_CUTOFF_MSAT)
+        self.assertEqual(reversed_fee_msat, budget.fee_msat)
+
+        # test value below cutoff
+        invoice_amount_msat = 1000
+        budget = PaymentFeeBudget.from_invoice_amount(
+            invoice_amount_msat=invoice_amount_msat,
+            config=config,
+        )
+        reversed_fee_msat = PaymentFeeBudget.reverse_from_total_amount(
+            total_amount_msat=invoice_amount_msat + budget.fee_msat,
+            config=config,
+        )
+        self.assertEqual(budget.fee_msat, config.LIGHTNING_PAYMENT_FEE_CUTOFF_MSAT)
+        self.assertEqual(reversed_fee_msat, budget.fee_msat)
+
+