Swamphen,
The 5% rule allows you to go over the stated limit of a class by a MAXIMUM of 5%, (Let's say it's down to a heavier brand of rivet being used) and remain treaty compliant!
So a BB of 35000 tons can actually be up to 36750 tons.
Or a DD of 2000 tons can actually be 2100 tons.
However the agreement is that we don't constantly MAX out on the weight, it's just a little WIGGLE room, to keep a design the way you want it.
So, you may well "need the steenkin' 5% rule!!!"