Okay, the 1 on empty ball was hard to see but if you take the ATM from the GTM it doesn't give much more and unless I'm mistaken ( which is possible ) that's how you get the max ball KGS.........I think !?!
Looking up GTM seems correct if your Tow ball weight was higher and you under ATM be less on the axles.
Gross Trailer Mass (GTM)
The GTM is the maximum weight that the caravan wheels can collectively impose on the ground. In other words, the wheels’ share of the ATM figure (remembering that the ATM is the coupling download weight and the weight of the wheels imposed on the ground added together).