"Or would i just have to say ok its a 45cal so .45 X 2 = .990"

Better check your math there. .45 x 2 = .90 (LOL)

I would bet moneythat is what you should use, a .90 max. No DOW official is going to measure the actual diameter of your gun, he will just look on the barrel and see that it is a 45 caliber. Then he will measure the conical and if it longer than .90, you would be in violation. For a 50 caliber, 1.0 inches.