The base of any 6.5mm bullet takes up the same amount of space, whether it's inside or below the neck.
I recently measured the water capacity of fired .260 and 6.5x55 brass, with a 120-grain bullet seated to the SAAMI overall cartridge length for each round (2.825 for the .260, 3.15 for the 6.5x55) and the difference was indeed about 2 grains.
It doesn't matter if the bullet is a 100, 120 or 140-grain. The difference in powder space inside the case will be the same amount, as long as the same bullet is used.
