Weight percents have ZERO basis in scientific fact, simple as that. Anyone with the ability to grasp basic logic should be able to see that. So lets see my in shape super conditioned 50 mile horse at 900 lbs can carry less weight than he can now soft and lazy at a 1000 lbs ? Really ? So an 1500lb draft horse with arthritis and a bad back can carry more than an 800 lb arab ? Wow good to know.
FACT over a 10 year study at tevis, (100 mile very hard endurance event) study found ZERO correlation between horse weight and rider weight in completion %, winning %, or pull %, up to and including riders at 32%.Confirmation and preride condition did.
FACT in an era when the average horse was barely 14 hands and probably 800 on a good day. A "Horse weight" was a unit of measure equal to 200 lbs. Used by people who's livlyhood depended on sound horses.
In that era the actual combat horses again 14 hands, were carrying armor and armored soldiers into combat. The giant chargers are a myth. And only used in parades. Actual war horses based on actual stored horse armor again, 14 handish.
Percents, Not a rule of thumb, Not a guideline, Not anything. Simply regurgitated nonsense. Has as much basis in the real world as saying a brown horse can carry more than a white.
Now Im not saying that horse can or can't carry you or anyone else, what I am saying is that his weight has nothing to do with it.