I don't know where the previous post is but the answer is straight from physics. Determine the center of gravity for the weight behind the rear axle. Measure the distance from the center of gravity behind to rear axle to the rear axle. Multiply the weight behind the rear axle times (the distance of the CG from the rear axle) divided by (the distance from the rear axle to the front axle). The difficulty will usually be in finding the CG and knowing the weight behind the rear axle. If the definition of center of gravity is not clear, I can define it.
2014 Palazzo 33.2
Cummins 6.7 ISB, Allison 2100 6 Speed