For several generations both S&W and Colt regulated their fixed sight guns for a 6 o'clock hold at 25 yards with standard weight loads
6:00 hold on what size target? 6:00 doesn't work unless the load, distance, and target size are known in advance; that is, you can't use it as a general method of sight-in or aligment, as 6:00 will differ with distance and target size.
You can come up with a reasonable point-blank range, but that again depends on the size of your target. If you are shooting at a paper plate, you can probably adjust your sights to that you can hit the plate by aiming at the center of it, from any distance out to 100 yards. If your target is a match box, then your point blank range, with a center hold, might be no more than twenty yards (if you can clearly see a match box at twenty yards!).