I'm trying to get an estimate on my ROI. (Nothing in lending club at the moment, but highly considering it.)

Let's say I put $5,000 in, I can easily calculate my return at the end of a 3 year period. However, I'm having trouble calculating the return if I reinvest the entire payment I receive each month. I created an excel sheet that shows the interest, term, principle, and monthly payment. The term is at 36 months. Below it I put a box that takes the original $5,000 principle, and adds to it the payment received. So know I have $5,000 plus the payment I received. (Interest and principle) I recalculate the interest based on 35 months left. In the end, it doesn't add up. So I've obviously miscalculating something. Maybe I need to subtract the total principle paid within that month from my invested amount?

So $5,000 up front, following by investing everything received each month up to 3 years. What do I have now? And is there a way to show a month by month chart?