New to BP - question about Seller Financing

[Joseph Parker]

Hello everyone! New member here - have found many useful tips & strategies on this site, but was never a member prior to today.

I have a question about seller financing calculations regarding a deal that I'm currently working on.

I'll spare you the full details of the deal, but I've essentially been able to get the seller to open up to a possible seller financing deal that would get me a great cash on cash return of near 20% on a good, solid property on the busiest street in town.

Here's where I'm somewhat stuck (and I hope you can follow). The latest offer we're working on is a purchase price of $400,000. $80,000 down payment due to seller. He wants 10 years of monthly payments around $1,650/mo, then a balloon due at year 10 of $180,000. So total of all payments due to seller would be roughly $458,000. 

Comparing these terms to the best financing I've found through a bank ($320k loan, 5.5% fixed for 10 years then refinance, amortized over 25), with same purchase price and down payment, results in a lower monthly payment by $315/mo, plus my principal balance after 10 years of payments through bank financing would be $240,000, compared to the $180,000 balloon that the seller wants at year 10. 

Given the seller financing details above, I know that an amortization table should show me paying $140,000 principal and roughly $58,000 interest over the 10 years of monthly payments, but how can I calculate the exact amortization schedule with principal/interest breakdown per payment? Guess and check base don various interest rates and amortization period? I'll also post this in the loan section but was hoping someone may have done a deal similar to this in the past and could help! Thank you all!