Owner carryback question...

[Scott Nipp]

I'm looking at purchasing a quad and wondering about possibly getting a 20% owner carryback and then only putting 10% actual cash down.  I'm not really clear on how to calculate the owner carryback and what to offer as a percentage rate on that portion of the financing.  Additionally, should I do the carryback for the full length of the note or possibly do that portion for only 10 or 20 years instead of the full 30?  

Any advice or suggestions on how to structure, calculate, and pitch this to the owner?

Thanks in advance.