I'm sure there are expenses they've been shouldering that you're getting a benefit from. 

Property APurchase Price - $340,000Total Rental income - $4100 (1200,1200,1700)Rental income less expenses - $3567Tax - $5495P&I - $1777Ins - $2000Mortgage (including costs) - $2401Cash Flow - $1166Property BPurchase Price - $439,000Total Rental Income - $4600 (1700,1700,1200)Rental income less expenses - $4002Tax - $5672P+I - $2294Ins -$2000Mortgage (including costs) - $2933Cash Flow - $1069Is there anything I am missing?

Then when I found something, I could run the numbers myself, but my contractor would weigh in on rehab expenses.

In addition to what Bill E. said, I try to get a good idea of what expenses could come up soon.  

With expenses of around $740 a month and renting at $1k, I would be bringing in $260 a month or $3120 a year.  

I also play with projected expenses to see what the returns look like if expenses come in higher than expected.  

I'll want to separate my personal and business expenses from the very beginning and I will be opening a business checking account and business credit card to help me do that.

It's getting too competitive and too expensive to pick up older homes to flip.

I teach my clients to have all the expenses on auto withdrawal  ( mortgage payments, monthly assm, if needed: tax and insurance) and have the phone number of few handymen on speed dial.

Also keep in mind I did not include any closing costs or payments and expenses you will have during the holding time while you rehab.