From my experience, the 1/4" thickness is ideal for big bore pistols. Now if you are holstering a Walther PPk, then you can get away with a thinner belt. It basically depends on the rig. The entire rig should be made for the specific pistol. Very important if your life depends on it.
I am sure there is probably some Trigonometry that could be used to determine the an exact radius, but I'm no rocket scientist. Ideally the radius for each person's body type is different, but that is not possible to obtain. I just have found one that works for the different lengths staying to the rule, smaller waist sizes require a tighter radius, larger sizes require a larger more gentle radius.
A frind of mine, Bruce Nelson, who was a famous holster maker taught me how to do this. Alot of makers today say that these types of belts aren't necessary because the straight cut belt will eventually conform. I think they just don't want to take the time to cut them. Contour belts feel more comfortable to me even after the break-in period for both. Galco does not hand cut their belts. They die cut for each of the sizes. My two clickers aren't big enough to use belt dies and I don't think most companies have the ability to die cut either. Galco is huge. Erik