You find the average cost per square foot of those properties, multiply it by the square foot of the subject property, then add/subtract for condition and other factors. 

You can estimate it by knowing the mortgage amount and subtracting it from the value of the home.

When I run the 50% rule with counting 10% for things like CapEx, repairs and maintenance, etc. and then subtract the mortgage payment I am sitting at about $100 a month.

If you stick with what's common on figuring a rehab then simply take your ARV x .65-.70 = {amount} then subtract your repairs, then subtract what you want to make on the deal.  

Subtract 6% for commision and I'm at $129,015. 4.

From that we have to subtract from the capital reserves.

Subtract out the rehab costs and your built-in fee and get a mortgage amount for her.Then get on your white horse and offer to solve all her problems, take away all her anxiety and headaches, AND produce an monthly income for her.

You take the value of the home and subtract it by the remaining mortgage on it.House Value-liablities___________=Equity

If not, then deduct another 7-10% for property management.Take the gross yearly income, subtract all the expenses I mentioned, and you have your NOI (net operating income).

Subtract the principal balance of the loan, along with all costs of the sale transaction for your NET gain.