I suppose I'm looking for a 'guestimate'. I understand your chalk test but don't really have the time to do it, and if you think about it, it could take multiple tries. It seems to me that 65 psi in all 4 tires (which would support a load of 9000 pounds) for a load of 3000 pounds ride is going to be unnecessarily harsh on the trailer, (which has no suspension), the horse and maybe even the truck. Obviously underinflating has its own problems, potentially more serious than a rough ride. I can ask the owner of the trailer (my daughter's riding coach) why she chose 45 psi but I don't get the impression she has put much thought into it.
The trip this weekend will only be a few miles. Next month it might be 50-100 miles and I may be able to get a better feel for how much air they need. And I may have time to do the chalk test.
You are right;
Max rated load at max pressure is exactly that.
If you are carrying max load, SURE you need max pressure, but things are probably marginal at that point.
If you drive around with max pressure at much less than max load the tires will be harsh and skippy.
Pilots of small private planes KNOW about nose wheel inflation pressures and HATE skippy nose wheels.
There are load/inflation tables for all tires; Goodyear, Firestone etc web sites have them.
225x75R15 is the most common 2 horse trailer tire, typically load range C or D.
The bigger ones on 16 inch rims are usually 85 profile - then 19.5 rims, but we're getting into the 6 and 8 horse trailers.
The "RIGHT" way to do it is to weigh the rig axle by axle, which is also useful to figure if one axle is carrying more than it's fair share.
The rear should carry SLIGHTLY more than the front, NEVER less (stability, sway).
As to what a 2 horse trailer "typically" weighs; Figure 3,000 lbs., I have a lighter all aluminum one that is 2780 with small tack room. Very few of the all steel ones are over 3400.
Med/large horses 1,000 lbs each.
With tack, hay and maybe water (10 gallons ?) for a one day show you will probably be pulling ~4,300 with one, say 5,500 with two.
Of COURSE it depends on the breed, 1400 lb warm bloods are not uncommon these days (-:
DO carry water to shows and trail rides a) It isn't always available there b) the horses won't always drink what IS available, so water from home is the way to go c) IF you get stuck in traffic, construction, truck problems, whatever you want water on board.
PS I know, my trailer is not "ALL" aluminum; the coupler and axles are steel, etc.
Folk use "all aluminum" to mean frame, floor, sides and roof, e.g. not aluminum skin on steel frame, not wood floor, not fiberglass roof.
