$6200 minus the expense you listed  and subtract another 8% each or so for vacancy and repairs gets you to around $3300 a month cash flow. 

(I arrived here by subtracting the new payment of $690 for the original payment of $815)Now we can see if spending the $8,500 is a wise decision.Brad will get paid $125 a month for 20 years and it’ll cost $8,500 to do it.Let’s put that into the calculator and solve for I/YR:N=240 (20 yrs), I/YR= ?

THe wrinkle is that there is a lot of rehab to be done, so you need to subtract that figure as well, to at least come up with a baseline value. 

After subtracting depreciation on the units themselves, which on mobile homes is a very real expense, I don't see much cash flow.  

Then of course you subtract rehab costs and wholesale fee but I really just want to dissect this 70% rule and understand how, why and when it might slide up and down?

Using the 3bedroom what would be the subtracting price per bedroom?

Look at total expenses and subtract from your gross rents.

A reasonable way to do this in many areas of the country is to subtract about .1% from the 2% target for each $100 that your rents are above $500.

Got some leftover padding at 20 cents per sq ft.It's pretty much been broken down to me as:$1,000 floors (materials, demo and labor)$1,000 paint (or subtract $300 if I supply my own paint)I'm OK with the floors but $700 for labor only to paint 1,500 sq ft (flooring area) has me thinking about this more.

From there I will take 70% of that and subtract my fee and a little in repairs ranging from 5-10k.