ok first thing you need to do is flush all the percent business out of your brain. let me ask you this, you say you are 260 lbs, you think you can wear body armor, ammo, helmet, and an 80 pound ruck like a 160 lb male infantry soldier can all day ?
Bone, fitness,, back and general confirmation determine what a horse can comfortably carry, NOT WEIGHT. That 20 % nonsense was created by an idiot bureaucrat army officer, who couldnt lead a cavalry unit out of a wet paper bag, he later became a full General and his manual has been getting quoted like its the word of God ever sense. IT is complete 100% malarky