For steel plates and bowling pins I practice on a 6" paper circle at 25 yds. 6" target printed on 8.5x11 paper. If I can hit the target most of the time, I am comfortable with the gun for competition. If I can rarely hit the target, then I need to adjust the sights, the load, or my shooting.
At our bowling pin shoot we have a paper target set up to the side. If a competitor empties a mag or two without knowcking down pins we have them shoot the paper from a rest. Surprising how many folks show up to shoot with no idea that their gun is shooting 12-14" off point of aim. I like to know that before I go to competitions.
For speed steel we normally have targets set much closer, and much bigger. When we get to a set with a 10" target at 25 yards, many folks are grumbling and reloading. If you are shooting 6" at 25 yards with some regularity, no big deal.