Damn it, I made a mistake.
S = r*theta where r ~= 9.0 (rim RADIUS.) + 2.0 (tire sidewall) and theta = 0.5*(pi/180) = 20*8.726E-3 = 0.096 inch ... 2.43 mm.
Rule of thumb is 1.0* rear camber gives approx 5mm of top clearance.
Skiddish, Surprised to hear that you could run that much 3.0-3.2* rear camber and get pretty good tire life. For reference, I'm using summer tires (Bridgestone S04). Can you please tell me your full alignment spec? I think I'll drop off my car to Dinan tomorrow.