I normally enter a value about halfway between the take-off CG and the ZFW CG values on the load sheet. 26% is typical. 5% is the forward limit = highest stalling speed, so the worst case for the buffet limit display.
(Stall speed increases with forward CG, as the tail carries less of the load/provides more down force, which then has to be taken up by the wings, i,e a higher angle of attack for any given weight.)