The 20% rule is bogus! Don't follow that, it all depends on your judgment, how well you ride, and on your horse in particular. Is she fine boned or heavy boned? Does she have a well built topline, strong coupling? Is she conditioned and in shape? Do you feel like she has a problem holding you? Can you ride nice and balanced without flopping around? Do you ride lightly? Does your tack fit well? Have you considered a pad that will evenly place your weight on her back?
THE 20% rule is used for static wieght like you would get with a pack saddle. I have put 150 pounds of salt on a good stout horse and seen them struggle with it because it doesn't move with the horse and throws them off balance a little easier.
Yep it all depends on the rider. I was talking to my trainer about this because I am a bit of a bigger girl but at the barn I use to be at I was the most experienced rider and was being asked by the owner to try out smaller horses to evaluate them for potential homes. Long story short, she told me that she looks at it as how good of a rider are you, are you dead weight, or are you moving with the horse, that makes a big difference.
Things like special padding to help distribute your weight as well as properly fitting tack come into play as well along with what kind of build your horse has.
Good luck you on the weight loss, let us know how it goes!
I have always heard people say, even some of the highend trainers around here, that they would rather put a 300lb rider that is balanced and has a light seat on their horse, then a 120lb rider who bounces all around, the lighter rider will do more damage.
I'm 270 and have ridden horses as little as 15.2 hands and 1100 pounds with no problems. I judge how I ride based on the horse. Say I was riding a 1100 tb, I wouldn't do any jumping, long trail riding etc. But when Im on my 1400lb gelding who I've been riding for 13 years, I know I can do anything I want with him. It really all depends on the horse. I rode a big, but lightly build warmblood once who had trouble with my weight, but on the opposite side I also rode a 14.3 hand spotted draft pony, who could haul me around on trail rides and everything with no problem what-so-ever.