I think the 20% rule is a good guideline, but its much more important to look at the overall conformation, fitness and bone structure of the horse.
I ride a 17hh, 1600lb spotted draft for work, and even though my weight + tack under the 20% rule is a min of 1050lbs horse... this horse is MUCH larger and I would NOT put anyone larger than myself on him, I am definitely at his upper weight limit not only because he's 5 years old and growing but his conformation is just plain terrible. Myself on him
He has gained weight since this picture was taken (in July) and probably grown some, but this is the only pic I have of him from the side (excuse his filthyness... he's a pigpen):
As you can see, he's got a LONG back and at 5 its already curvier than I'd like to see. His feet are small for his size as well (he's a draft size 3 shoe), he has a very narrow chest as well and a narrow frame in general. Though he IS a draft horse, he's a pretty terrible weight carrier.
On the other hand our new horse is the same breed (spotted draft) is 16hh and probably only about 1300lbs (he needs about 100lbs of weight, we got him from the dealer skinnier than I like a horse to be). He's 6 years old and has MUCH better confirmation than our 5 year old for carrying weight. He has a wide-set frame, short cannons and a compact back. His shoe size is a draft size 4 so bigger feet than our other guy as well. I don't have a pic of him from the side, but you can see he's just a TANK:
When he gets up to a good weight around (1400-1450lbs) i'd have no problem having someone as large as 350lbs on his back simply because his conformation and build dictates that he is a very good weight carrier.
Haflingers are also great weight carriers. My haflinger is a fit and healthy 900lbs at 14hh, but he carries me well because of how he's built. Thick cannons, short back (my 18" dressage saddle is as large as he can take) and he has excellent confirmation for moderate weight carrying as well. HOWEVER, because he is of the "sportier" variety of a haflinger, his upper weight limit is probably around 225 with tack... so I'm almost there.
Another haflinger I ride regularly is 13.2hh. Although a fairly obese 1100lbs his frame at a healthier weight would still be around 1000lbs because he's built with a wider frame and stockier bones than my haflinger.
As he is right now I think his max weight is around 300lbs, but if he were in better shape, I think he wouldn't have a problem with 350lbs.
Obviously I'm no vet... but what any well educated horse person knows about bone structure, confirmation and fitness you can make an educated guess as to what that particular horse will be able to handle. Of course, there are individuals out there who don't take that into account... but we can only educate and not humiliate!
If you as an individual, feel you are not as fit and as balanced as you would like to be as far as riding goes, than select a horse that is not at the lower end of your weight range, but rather one that will be able to support you well as you reach your fitness goals. A stout draft or draft cross like my 6 year old horse at work, or the stockier haflinger... would be a good choice if you are 250lbs+