Frosty Hoar,
If you are predicating the TOPL on an MAC of 25% to flight plan without a tech stop, you have to be sure that the aircraft will be loaded in such a way that the CofG really is 25% (or within a certain parameter) on the day. This may be possible by loading the bags in the rear of the aft hold and 'filling up' forwards, stowing catering in the aft galley, passenger seating plans, etc.
The fundamental point is that the pilots will be presented with a loadsheet that shows them where the CofG is, and if it isn't 25% or thereabouts they won't be able to use the extra performance.