Your calculations sound correct. I think the tire and wheel calculator will help you with tire/wheel assembly sizes so you can measure adequately. Goody - got that link handy? Its JAVA based and helped me a lot fitting 315's on the rear of my car.
I don't know how you plan to drive the car but if you are going with a tall profile like a 50, 60, 0r 70 series, you may want more than .250 clearance on the spring.