[Qt] add defaultConfirmTarget constant to sendcoinsdialog

- replaces some hard-coded values for the default confirmation target
- also simplify code that is using the new constant

src/qt/sendcoinsdialog.cpp

@@ -590,12 +590,12 @@ void SendCoinsDialog::updateGlobalFeeVariables()
 {
     if (ui->radioSmartFee->isChecked())
     {
-        nTxConfirmTarget = (int)25 - (int)std::max(0, std::min(24, ui->sliderSmartFee->value()));
+        nTxConfirmTarget = defaultConfirmTarget - ui->sliderSmartFee->value();
         payTxFee = CFeeRate(0);
     }
     else
     {
-        nTxConfirmTarget = 25;
+        nTxConfirmTarget = defaultConfirmTarget;
         payTxFee = CFeeRate(ui->customFee->value());
         fPayAtLeastCustomFee = ui->radioCustomAtLeast->isChecked();
     }
@@ -629,7 +629,7 @@ void SendCoinsDialog::updateSmartFeeLabel()
     if(!model || !model->getOptionsModel())
         return;
 
-    int nBlocksToConfirm = (int)25 - (int)std::max(0, std::min(24, ui->sliderSmartFee->value()));
+    int nBlocksToConfirm = defaultConfirmTarget - ui->sliderSmartFee->value();
     CFeeRate feeRate = mempool.estimateFee(nBlocksToConfirm);
     if (feeRate <= CFeeRate(0)) // not enough data => minfee
     {

src/qt/sendcoinsdialog.h

@@ -23,6 +23,8 @@ QT_BEGIN_NAMESPACE
 class QUrl;
 QT_END_NAMESPACE
 
+const int defaultConfirmTarget = 25;
+
 /** Dialog for sending bitcoins */
 class SendCoinsDialog : public QDialog
 {