I typically subtract 3%, of the ARV.Realtor Fees: What is the commission you are willing to pay your listing agent (unless you are the listing agent) and the buyer's agent.

Should I still calculate purchase price using the formula but with a higher % or is it better to subtract all repair expenses from projected ARV?

And make adjustments for unique features and amenities by adding and subtracting.

If it were just the 2 units the 2 couples could agree on a reasonable rent and then subtract that out of their share of the profit. 

Lastly, only buy them if, by subtracting their mortgage, plus half of their rent return, you still result in a positive cash flow!

I typically subtract 3%, of the ARV.Realtor Fees: What is the commission you are willing to pay your listing agent (unless you are the listing agent) and the buyer's agent.

You then subtract what the estimated debt service would be - say $800.

I can add/subtract/multiply/divide.

The home you live in is NOT an investment...it is however your biggest cost.This is a simple math problem.If you put money in a property and leave it there, it is a negative number = Your cashIf you get money out of a project, it is a positive number = your profit or cash flowIf someone else puts the money in a project, it doesn't count as positive or negative = leveraged funds...unless, you have to pay for those funds = loan interest.However, if someone else pays for those "interest costs", they don't count as positive or negative = Cash flowJust apply all the cash moving in and out of your deal(s) to one of the above definitions, add (or subtract) all the numbers as indicated above, and you will see either a total that is positive...or negative.If it's positive, you are making money.  

Subtracting the $4k in closing for the first purchase, the $2k in closing for the sell a year from now, and the $8.4k in selling fees from a conservative sale value of $140k leaves you with $125,600 minus your purchase price of $118k and you pocket about $8k.