I've wanted to build this into a web application for a while, but the idea always ballooned into wanting to do a YNAB or Mint competitor with debt calculator as just one feature. And while possible, building something like that as a solo founds seems daunting!

Anyway - check out this google sheet by Vertex42 which has some different paydown options, instead of just Snowball. Maybe something you can incorporate, if you haven't already.

http://www.vertex42.com/Calculators/debt-reduction-calculato...