Divide fee calculation logic into parts

src/guide/receive_onchain.md

@@ -290,12 +290,30 @@ result, err := sdkServices.Refund(refundable.BitcoinAddress, destinationAddress,
 <div slot="title">Dart</div>
 <section>
 
+To calculate fees, you'll need to detect if channel opening fees are needed first and then calculate the fee for the amount you'll be receiving.
+
 ```dart
-int calculateChannelOpeningFee(int amountSats) {
+int calculateChannelOpeningFee(int amountSats) async {
+    bool isChannelOpeningFeeNeeded = await isChannelOpeningFeeNeeded(amountSats);
+    return isChannelOpeningFeeNeeded ? calculateFeesForAmount(amountSats) : 0;
+} 
+```
+
+How to detect if open channel fees are needed.
+
+```dart
+bool isChannelOpeningFeeNeeded(int amountSats) async {
     NodeState? nodeState = await getNodeState();
     int liquidity = nodeState.inboundLiquidityMsats ~/ 1000;
     // Check if we need to open channel
-    if(amountSats >= liquidity) {
+    return amountSats >= liquidity;
+}
+```
+
+How to calculate the fee for a specific amount.
+
+```dart
+int calculateFeesForAmount(int amountSats) async {
     // We need to open channel so we are calculating the fees for the LSP
     String? lspId = await getLspId();
     LSPInformation? lspInformation = await fetchLspInfo(lspId!);
@@ -309,7 +327,6 @@ int calculateChannelOpeningFee(int amountSats) {
     int channelFeesMsat = (amountSats * setupFee ~/ 100);
     // If the proportional fee is smaller than minimum fees, minimum fees is selected.
     return max(channelFeesMsat, minFee);
-    }
 }
 ```
 </section>