Quickie, investment #'s question.

I've read a ton of investment numbers here, but I need to do this for clarity.

I need to know what % the mortgage should take of the incoming rents. Should I include tax in the mortgage when calculating this?

This example is close to the reality i'm looking at. I'm moving the numbers a bit.

Im looking at a 2 family for 300k it takes in 3k a month in rents. Thats about an 1800 mortgage payment at 6% not including Tax etc.

If I offered 250k this would be a 1500 mortgage payment at the same 6% rate not including tax etc. while still taking in 3k

Question: would purchasing the home for 250k put me in that area where rentals should = 50% of mortgage payment, or am I missing something.

TIA.