There is an app. It's called you brain. If paying multiple cards, with either the avalanche or snowball method its minimum balance on all except for 1 card where you make all the payment you can regardless of minimum.

There isn't any math to do or worry about. The only info you need are either balances, for snowball method, or interest rates for avalanche method.

It seems like you are just struggling with how to implement the program because they are very straightforward.