@Aviv Berkovitch
My understanding may be the same as the previous response, but I am not sure exactly what Stephanie said.
First, whether you can use projected rental income for a property you are planning to purchase is up to the investor that is purchasing your loan; some may allow, others won't. I could not tell from Stephanie's response whether she was including the property you are attempting to purchase in the DTI.
For the DTI calculation, using rental property you already own and have a rental history on your tax returns, my experience is that my net annual rental income from the tax return plus depreciation, plus mortgage interest is divided by 12, then PI is subrtacted. If the answer is positive, then that amount is added to my total income to calculate DTI. If the amount is negative, then that amount is added to my debt service to calculate DTI.
In the absence of a tax return, but with proof of ownership and a long term lease, the lender will use 75% of scheduled monthly rent and subtract HOA fees and monthly PITI. If the answer is positive then it is included in gross income, if negative it is included in debt service to calculate the DTI.
I admit it has been at least 13 years since I acquired my last rental property, so my understanding of the DTI calculation mechanism may be outdated. I disagree with your assertion that a positive cash flow from your first rental property does not change the DTI you had before you acquired the rental property.