Most comon is around 45, but to get a real number you need at least to have the crown height for the fork you will need for the frame you have, and the headset you will run, and the head tube angle. Also, since it doesn't hurt anything to have more info, you may as well know the actual inflated diameter of the wheel and tyre you will run, though that starts to get picky unless a few similar small numbers get added up.

Wait a second, are you running the 650 on a 700 frame. Otherwise why aren't you just measuring the offset on your current fork. What kind of brakes are you moving to.

Your question isn't really a framebuilding one, unless you are planing to make a 650 fork that will fit properly in the place of the 700c.