What % to buy a rent to own note?

[Kale Stephens]

I met a guy who inherited 3 rental properties and 2 rent to own properties. His dad was a slum lord in Abilene, TX. These houses are all in rough shape, but seem like a great opportunity for some BRRR properties. The two properties that are rent to own have about 4 of the 8 years left on the note. The contract has a set monthly payment, and does not state what the interest rate on the note is. In this case It seems like I am buying the note, not the property, so I feel like my offer should be based on buying the remaining 4 years of the note. What percentage should I offer and how do I justify that to the seller? Thanks, Kale