I’m sorry, you cannot – CANNOT- reduce weight-carrying ability down to just a simple 20% rule. It does not take into account so many other factors.
Let’s look at it in human terms. It’s like telling a 600 lb man he should be able to bench-press 150 lbs simply because he weighs 600 lbs. Fitness doesn’t work like that. The 600 lb man might be barely able to lift a big bag of dog food because he is so out of shape. He is fighting against his own excess weight just to lift that bag. On the other hand, a skinny, ropy, 130 lb man who is extremely fit and pure muscle might be able to take that 50 lb bag of dog food and go for a run- even though the bag is roughly 40% of his weight! He is fit, his muscles are used to the work, and he knows how to use his body. He’s also not fighting excess weight to do a job.
What I’d want to know:
*What type of riding are you doing? Is this rough trails, jumping, dressage, or walking around an arena for 30 minutes once a week? This matters. It’s the difference between someone who goes to the gym every single day and somebody who goes for a walk in the park on Sunday afternoons.
*How much work does this horse normally do? Does he get used hard all day, every day, or is he just pulled out of the pasture once a week? How fit is he, from a scale of couch potato to gym rat? If he were a person, would he be the guy who lifts heavy weights each day, goes jogging after sitting down at a desk, or the dude who gets winded walking across a flat parking lot?
*What is his conformation like? A short, stocky, close-coupled horse like a bulldog-type QH is going to be a better weight carrier than a tall, leggy ASB, especially one who is not fit.
Now for the bad news – I don’t think your ASB will be a good fit for you until you are fitter. The conformation difference between the two breeds are just too huge and too severe, and ASBs aren’t great weight carriers to begin with. They tend to have thin bones, long backs, and are thin in the chest- all of which just don’t equate to weight carrying. Stick with the QH for now, but pay attention to his health and make sure you are warming him up and cooling him out properly each time you ride. If he has heavy bone, is in good fitness himself with solid muscle and a good topline, there’s no reason he can’t carry you as long as you keep an eye on his health. If you’re not already, pay attention to his legs and joints and any heat or swelling that occurs after you ride. If he’s uncomfortable for days after you ride, it might be a sign that he’s being worked too much too quickly.