I just saw this and thought what effect it might have on ones telescope making efforts. Conventional wisdom was to ring support the mirror and thus even today molded blanks often have a ringed edge. Deformation is toward a shorter focus paraboloid and still used by european amateurs. I believe it was Shapley that applied finite analysis and came up with the 70% zone for perforated optics. The idea is to have negligble deformation not controlled deformation. At 50% pointed straight up and allowed to deform the result maybe a longer focus paraboloid but what if you don't want a longer focus or a paraboloid? Or if you don't want it pointed straight up? I have seen the finite analysis for the 8.2 meter Keck and its wiffle tree is based on appoximately 70% zone centering as determined by the analysis. Since time and science has proven the 70% zone, I would wonder where I would find the very special circumstance that warrants a 50% zone. One of my first mirrors sits on three points at 67% and holds better than 1/10 wave in any orientation and it is 40 years old. As the saying goes, it is hard to argue against success. -Rick