Faye, the 20% is generally correct or close to correct when used as a basic guide. However, when actual studies are conducted, it would be more accurate to determine weight carrying ability based on bone structure and fitness rather than weight. For example, a 15.2hh, stout quarter horse with fantastic bone, great conformation and ideal fitness may weigh 1100lbs. However, a 14.2hh pony with terrible legs, a weak back and a few hundred pounds of extra fat could, realistically, weigh the same. Can they both comfortably carry 220 lbs? Absolutely not. The pony would struggle significantly because of his lack of fitness and poor conformation, though the quarter horse would probably be fine with more than that figure.
If a horse isn't a conformation trainwreck or vastly overweight, the 20% rule is fine to go by. I use it with my personal horses. All I'm saying is that it would be more accurate to take into account the horse's level of fitness and conformation, especially if the rider's weight is questionable.