Need help with wheel selection and stance

[T Bell]

I've found some BMW wheels that should fit my Cutlass. They're 18x8 fr and 18x9.5 rr. Of course the backspacing won't work. I'll either lengthen my axle tubes or buy a ford 9 to fit. My main thing is how to make my wheels fit my car, stancewise. I've seen many a camaro with the right wheel and tire combo. I want to nail mine but lack the design influence to make it look right on the 1st try. Isn't there a formula of some sort that relates wheel diameter and tire sidewall height to the car''s wheelwell?