The rule of thumb for weight on a 4WD tractor is 40% of the total tractor/implement weight on the front wheels, 60% on the rear. One way to measure is to get your tractor equipped with the implement you want to use, and drive over a truck scale, one axle at a time.
Remember that if you need more weight on the rear, for example, that 200 pounds of ballast (water or steel) in the rear tires adds 200 pounds to the rear axle weight only, but 200 pounds four feet behind the 3pt has more mechanical advantage (leverage), and will actually lessen the weight on the front axle while transfering more than 200 pounds to the rear axle. Calculations are beyond the scope of this post. An engineer friend can help you with formulas quite easily.
I bought a 700 pound elevator weight at a junk yord for $30 to hang on my 3pt. easy project, easy on, easy off.
Good luck,
JRPoux