You father is correct, unless your plane is a canard configuration.

The reason is that during flight the CG moves toward the tail as the fuel is used.
There is a limit for that aft position of the CG, beyong which the plane becomes very unstable.
Then, we measure the location of the CG for the worst condition (empty tank), when the CG is all the way back, but not beyond that limit.

This applies for glow, gas or any liquid fuel powered model.

Check this article out:

http://www.rcuniverse.com/magazine/a...article_id=601

Regards