Once I subtract additional expenses (see above) I'm left with an average of 25-35% net gain over LTR.

Subtract that number minus the what you plan to sell the home for.  

There's your total offer price...be sure to subtract any fees from that.

You take the 70%, then subtract the amount for repairs, and you come up with a number of $40K max offer.

-his property value with subtractions for repair 102-13(foundation)-10(sewer)-10 cosmetic updates -5(roof)-64k-he owes 92kI approached him with an offer of 92k.

Take that number and subtract your estimated rehab costs. 

In Rochester, Buffalo, and Niagara Falls, NY, I can find SFR's and Duplexes that will bring in gross rents of 2% of the purchase price, and I can calculate 50% of the gross rents to be counted as "expenses" (like Mike Oh's rule) and subtract the debt service and be able to cashflow $100 per month for each $50,000 valued property.

If you can add and subtract, AND NOT RATIONALIZE bad numbers, market analysis is easy...but make no mistake about it, it is the most important analysis you will do.  

If you subtract out a few because they've been displaced by the SFHs, and add back, say $50,000 for each SFH (have they been parceled-out on their own land?)

And, you need to subtract the HOA fee from the rental income, because that is not your money.