I need math Help on a seller financing deal

[Michelle Mapp]

I am working on a seller financing deal and I am looking for some resources on the math/formula part of it.   I need to create a detailed spreadsheet with the full amortization schedule that allows for quite a bit of flexibility - such as increasing the payments every 5 to 10 years, making extra lump sum payments at different times, etc.  

I am trying to start from a specific monthly payment and then work around the numbers to get to a point where everyone is happy.  

Does this make sense?