Well, I didn't know if such a spreadsheet was out there at all. Actually, it probably is ... but the question gave me an excuse to MAKE A NEW SPREADSHEET!
IYM: Dave Ramsey's Pro Rata Debts Spreadsheet
In a nutshell, Dave advises readers who are having trouble making debt payments to always send a least a little something — but to do it in a manner that's not preferential to any one creditor. This spreadsheet accomplishes that (I hope!) by computing payments for each creditor which are proportional to that creditor's share of your total debt.