:2cents: What you can learn by practicing at 20 yards is limited.
Much better to practice at 40. When you can shoot 10" groups at 40 yards, you will be well on your way to good accuracy. Those that develop and fine tune their skills, utilizing a 20 yard limit, when faced with shots beyond that distance are at square one and need to begin again. When you routinely practice at 40 yards you also learn about executing the closer shots.
If you were already proficient with the bow and were conditioning yourself to shoot an indoor league at a set distance, for example 20 yard 300 rounds, then it would make sense to practice at 20. For hunting, it makes no sense at all to practice at 20 yards. A 24 yard shot, a distance that all those 20 yard aces are going to attempt given the opportunity, is in the as yet unresolved realm, so you would wind up guessing at the worst possible moment, the moment that you need to be on top of your game. If you're unpracticed at 24 yards, you can't responsibly take that shot. It's a poke 'n hope.
The reason that there is a 20 yard hunting ethic is because mediocre and wrong practice leaves the majority in the condition of not being able to make the shot if it's over 10 yards.
After you get good at 40 yards, start practicing at 60. That's how you become a good archer.