It's pretty simple to figure out how your weight is distributed between your axles.
Let's imagine you have a 10 foot load that weighs 10,000 pounds. Let's also assume that your load isn't heavier on one end than it is on the other, meaning that it's own weight is distributed evenly among it's 10 feet. If you place that load halfway between your drive axles and the center of your trailer axles, The weight will be split exactly (more or less) in two, leaving 5000 pounds on your drive axles and 5000 pounds on your trailer axles.
If you then split the distance between the center of your load and the center of your trailer axles, that would equal 1/4 of the distance between your drive axles and your trailer axles. If you center your load on this 1/4 mark, you would then be left with three quarters of your weight over your trailer axles and 1/4 of the weight on the drive axle(s).
Does that make sense?
In my experience, I have found that the "1/4 away from the trailer axle" point (or 3/4 away from the drive axle point) is the best place to center your load's weight, all the way up to your maximum carrying capacity without going over any axle weight limits, assuming you purchased the proper equipment for what you intend to haul.
I'm posting a link to a simple drawing that very clearly depicts how to calculate a "moment in weight" or how the distance from center effects weight distribution.
www.pilotfriend.com/safe/safety/images3/10.jpg
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