I understand that you may think that spf is too much money. But, consider this. If you use spf then you would not have to vent the attic portion or incerase the size of your ac. What you suggested is not a bad approach, but if you check out some other homes that have spf in them you would see...
I am new to the forum. But... I am an insulator by trade. I would reccomend spf open cell because of the fact that it has the best resistance to solar heat gain. Also you would not have to have that area that is sprayed vented. I do realize that it may cost a little more but your comfort level...