Just found an interesting site for calculating caravan weight distribution.

http://www.racingaspirations.com/apps/weight-distribution-calculator

I used a front overhang of 25mm (assuming a 50mm ball is at the front), wheelbase of 3300mm & rear overhang of 2600.

Then i used a 1300kg ballast (van tare) and placed it in the middle but towards the back so that each front corner showed 40kg (van has 80kg ball weight).

Then used 1600kg (van max) and did same thing trying to keep 40kg on each front wheel.

Then 1300kg with 3 x 100 kg ballast to keep 40kg. 

Interesting exercise. You could even add water (fuel) tanks (ilo of some ballast) and see where best to place an additional one.

It's not to scale so you will need to check were the ballast is located by the figures at the top of the pic.