small steel plates are most often found between 12 and 20m, so split that distance, and my gun is set for about 15-16m. This means that I don't really need to worry about trying to compensate for any shots until we get into the 40+m range, and those are very rare. Set it so you don't have to think about it for most of the shots, and just make a point of compensating on those rare shots.
Haha, I have a friend who, shooting open, wanted to be able to drop plates at 25m with a perfect shot, so that's what his gun was set for. with his old open gun with a high dot, there was a much greater paralex, so the poor guy had to mentally adjust for something like 80% of his shots, all in the name of not having to adjust for those (rare) 25m plate shots... He has seen the silliness of his ways now.
Set your signs so that you will cover most of the every-day shots without having to think about it, and also know how your bullets behave at extreme ranges, for when those targets appear in a COF.