I agree with Wheel Nut. Like Juddian I remember 32 tons and 38ft and 40ft flats which were just too short to get a full weight of fruit on the floor. Depending upon what the load was and the trailer, the extra went over the bogie usually broken down into half height pallets, with maybe some handballed up the back if it was a 38 footer.
It was never, never stack the extra up the front because apart from overloading the unit that is where the swing/sway on an artic is the greatest.
I did write a speadsheet some 20 years ago to get somewhere near axle the weights for a Gray & Adams fridge and an Actros according to the position of each pallet, but it will probably take me a week to remember how to use it!