https://github.com/bitcoin/bitcoin/blob/5180a86c96bc05d2a731f70f36aae28ab5a3fad4/src/qt/sendcoinsdialog.cpp#L339

I think this is %1, not %2
{
    if(!model || !model->getOptionsModel())
        return;
    if (ui->radioSmartFee->isChecked())
        ui->labelFeeMinimized->setText(ui->labelSmartFee->text());
    else {
        ui->labelFeeMinimized->setText(BitcoinUnits::formatWithUnit(model->getOptionsModel()->getDisplayUnit(), ui->customFee->value()) + "/kB");
    }
}
void SendCoinsDialog::updateMinFeeLabel()
{
    if (model && model->getOptionsModel())
        ui->checkBoxMinimumFee->setText(tr("Pay only the required fee of %1").arg(
            BitcoinUnits::formatWithUnit(model->getOptionsModel()->getDisplayUnit(), GetRequiredFee(1000)) + "/kB")
        );
}
void SendCoinsDialog::updateCoinControlState(CCoinControl& ctrl)
{
    if (ui->radioCustomFee->isChecked()) {
        ctrl.m_feerate = CFeeRate(ui->customFee->value());
    } else {
        ctrl.m_feerate.reset();
    }
    
    
    ctrl.m_confirm_target = getConfTargetForIndex(ui->confTargetSelector->currentIndex());
    ctrl.signalRbf = ui->optInRBF->isChecked();
}
void SendCoinsDialog::updateSmartFeeLabel()