Little bit of light reading for you:
http://www.mgf.ultimatemg.com/group2...slip_angle.htm
I would be interested to know what vehicle you have, its power and supension set up? A starting point would be standard suspension/55bhp = 165 tyres all round
the Porsche has staggered wheels simply because the load on each corner is very different front to rear. They will be after maximum grip at the rear to tame the gross polar moment that is generated by the rear engine and be able to lay down the power effectively. I suspect that the fronts would have been narrower but the difference in the slip angles would have made it very twitchy and also they needed some big rubber to complement the brake performance.
Clive