There is no standard practice distance, only you can determine what is best. You should practice for how you plan to use your gun. Assuming your gun is for self defense, IMO, 20 yards is a bit extreme. In reality, do you plan on engaging a BG who's 60 feet away? I rarely go beyond 7 yards because I know that, statistically, that's the maximum range I will be using the gun in a self defense situation. I'd suggest bringing the target in closer, work that distance until you are comfortable with the groups you are producing, and then start moving the target out a couple yards at a time, each time increasing the distance only when you are happy with your grouping.
.45 is going to be expensive to shoot. You might have been better off getting a 9mm for the cheaper ammo.