Paypal plugin fee calculation

I searched the forum and didn't see anything about this. The Paypal fee isn't being calculated correctly, because Paypal calculates the fee based on the total amount you send to them (which includes the fee), not on the base amount. For example, if your base amount is $30.00 and the Paypal formula is $0.30+2.2%, then it is incorrect to calculate 0.3+2.2%*(30) = 0.96 and then send $30.96 to Paypal. If you do, Paypal will charge $0.30+2.2%*($30.96) which is $0.98. So you end up with only $29.98 rather than $30.00. Does that make sense?

So the proper calculation would be:

( ( $30+$0.30) / 0.978) - $30

Or in the general case where B is the base amount we want to end up with, P is the price in the configuration menu, Q is the Percentage in the configuration menu and F is the calculated fee:

F = (B+P) / (1-Q) - B

Thanks for your reply and I understand your reasoning, but still would like the calculation to match Paypal. Can you point me to what source code does the calculation so I can try to do my own mod?

Here's the new code if anybody else wants it:

$methods[$k]->payment_price = $currencyClass->round((($price_all + @$method->payment_price) / (1-(float)@$method->payment_params->payment_percentage / 100))-$price_all,$currencyClass->getRounding($currency,true));

But now when I go to checkout, it's showing the correct fee next to the payment method, but the amount shown under "Payment fee" at the bottom (and added into the total) is still the old calculation. Is there another place I need to change the formula?

No, it's definitely persisting. Anything else I can try?

That was it. It's perfect now. Thanks so much!

New code:

$payment->payment_price = $currencyClass->round((($price_all + $payment_price) / (1 - (float)@$payment->payment_params->payment_percentage / 100)) - $price_all,$currencyClass->getRounding($currency, true));