Debt to income ratio with investment property?

[Michael Osborne]

I currently own 3 properties renting for a total of $5100. Applying 75% of the rent as income leaves me at $3,825 in income from these rentals, the mortgage, taxes and insurance come up to $4,550 a month. When calculating my DTI ratio does just the difference of $725 could as debt?

This is our only debt and we plan to buy another primary residence in about 18 months. We live in a high cost of living area so are looking in the $600k range with 20% down. Our current W2 income is about $150k. Assuming a mortgage payment of $2800 on this property, would we meet the DTI guidelines for a conventional mortgage or not. We are long term buy and hold investors so do not plan to sell any of these properties.