Renting out primary and buying new house

[Jonathan Wiesniak]

I own a house in South Florida worth about $650k.  We are planning to rent it out and move to Massachusetts. I Have about $320k in equity.  mortgage all in is $2600 / month and a conservative rental estimate is $3700-$3900.

The tricky thing is my wife will move first, and I will follow her about 3 months later.  Her income will not count towards the new mortgage as she just finished up lengthy stay at home mom duties.  Im going to need the rental income to show for the mortgage as I think we will come up short in what loan amount we are looking for.    

So my question is, will the rental income count even before I have a lease agreement in place ?  And is it $3700 - 3900 x 12 months all multipled by 75% for the income calculation ?  

The point of this is, I wanted to move from house in S. Fl to house in Mass.  Not move to an apartment  in Mass. for a few months while looking for houses.