How lenders calculates debt to income

[Wilfred Mkilindi]

Hi everyone, have a question.
Lets say by DTI ratio is 42% the income part being from my full time job only and i happen to qualify to buy 1st rental that is renting for lets say $700/month with a cash flow of $250/month, 6 months later i want a 2nd rental but now i have a new loan (1st rental) on my credit meaning my debt went up as well as new income from 1st rental.
When lender is calculating my DTI ratio to buy a 2nd rental, income part will have my full time job and what part from 1st rental, is it the $700 or $250 cash flow? 

Thank you