and then later subtract this income from units that don't pay because of vacancy or credit loss by multiplying it?

Multiply it by 1.0025.

I understand the desire to make all the profit and to have a big multiplier by improving the whole area.

Also taking your example think like thisOKC -150k house rents for approx $1200 (i am being very general you may get more) with 25% down you have 38k tied up in the houseSD - 400k house rents for double so $2400 with 25% down you have 100k tied upif you multiply this out you could do 4 houses in okc for the money of 1 in SD and you would have rents of $4800 per month coming in.  

This will allow your money to be multiplied and to have continuous flips in your pipeline.

To get this number i multiplied 4800 (a months income) by 12 and divided by .10 the desired cap rate.   

Problem is the expenses, so(GSI - GSI*0.3 ) * 10 is a rational price:the 10 is a Gross Rent Multiplier and arbitrary; but reasonable range is 8-12GSI*0.3 is an assumed expense ratio of 30%; higher will be high maintenance costsso GSI * 70% * GRM of your choice = Purchase Price.If you get a GRM > 15 (I've see as hi as 19) the seller is in trouble and over extended in the debit+profits

The sale price was $430,000 ($53,750 Per unit, 7.66 Cap Rate and a Gross Rent Multiplier of 13.04).

A conglomerate that buys 10,000 units at a time can afford to make $50/month/door because of the multiplier effect, but becomes that much more exposed if there is any kind of generalized downturn. 

My REFI of Hold properties take place at the end of a series of flips repeating the use of the same initial cash from the first flip to generate a series of flips that multiply the number of simultaneous flips you are able to do by using the profits from the original line flips and subsequent added flips to generate those simultaneous flips, and each repeated set of simultaneous flips will generate a new horizontal line of Holds starting with the first one, generated from that level of simultaneous flips, and expanded through the use of....you guessed it, REFI's.