Farahmay, 20% rule comes from the time of the cavalry, talking about a certain type of horse rider combination, working HARD. There was a study done at Ohio University that is oft quoted to confirm those findings, but being as they studied a whole 7 horses, and they were pasture fit horses, I give it very little credence.
There is a huge study done of horses competing in the Tevis Cup, where the conclusion is that carrying up to 30% of the horses weight is not detrimental. I have issues with that one as well, we have gone from pasture fit to a level of fitness that is beyond most casual horse and rider combinations. I also can't find what % of the 700 or so horses that they studied were carrying heavier weights, so have no idea of the statistical relevance.
I think we will struggle to come up with any sort of formula that is a catch all across all breeds, disciplines, rider shapes and sizes, their abilities and choice of tack. Oh I forgot levels of fitness for both horse and rider, there are just far to many variables.
What can we say, obviously the lighter you are the easier you are to carry, right? Well not always, go out to a playground and find 2 kids that look equal in size and weight, try and find one that looks hyper, and one reading a book. Give them both a piggy back ride, then when the police and justice system let you out of jail, tell me if the quiet still one, or the hyper jiggly one was easier to carry.
Now look at this
One rucksack costs $50, one costs $450, which would you rather carry a heavy load in? The one that has suspension, or the other?
I wish people wouldn't jump on the 'high end' tack statement as being elitist, there is a lot of cheap, shoddy, poorly fitting, thrown together junk tack out there. Quality does not always mean spending $1000's my most expensive saddle is $800, and it is a quality saddle, it may not be the top of the range, but it is well made and comfortable for both me and the horse, and they have a fantastic customer service department, who helped me out lots even when I wasn't buying new.
Shop wisely, there are places to save money, and it isn't in your saddle, your saddle pad, your safety gear or your bit.
As to 300 pounds on a 1000 pound horse, well yes I think that can work, for walking work, short rides, as part of getting fitter. That assumes the right horse rider combination.!