The beauty in it is, it can be adjusted quarterly or annual as you see fit. 

You need to do the same thing an appraiser does - get comps, adjust the comps based on differences with your property and then come up with a value. 

A dear friend of mine is an adjuster who comes across properties as well.

I found it hard to get him to offer the package I put together and after some compromise and adjustment we agreed on an offer that we thought would be resonable. 

For instance the Denver opportunity may be somewhat lower return but it's close to home so should get adjusted to be more favorable.

Measure results and adjust.

I'm not sure there's a general formula to adjust the rule of thumb calculations though, if that's what your looking for.

The fact that you have "knobs" to turn as you adjust to changes in the economy is very comforting vs. many other asset classes.Best of luck!

You can always go back to the owner with an adjusted # if it comes in lower.

If you really want to do a quasi owner finance for your friend to pay off the property within 3 - 4 years, you could simply set a monthly rent that is high enough to accomplish that goal, do a 6-month purchase contract and reduce the purchase price each time by whatever he's paid, with a handshake that you would adjust the actual purchase price, if he ever decided to finance it in the middle of a contract term.