Wheel Rim Backspacing 40 Ford question

What backspacing are you 40 ford guys running on your rims with ford 9 inch rear? I have a set of cragers and with the full jounce I need a feeler gage to measure clearence. My tires are p245-60-15's.

 

57" drum-to-drum
245-60R15
15x7 with 3.5" backspacing

Good clearance all around

 

Thanks 40tudor for the info I was thinking I need at least 1/2 inch clearence all around. My back spacing is 4 or 4 1/4 I need to measure again to make sure.