Always balance a plane in it's most tail heavy possible configuration, as that will set the worst possible case.

For most planes, that means ballancing with the tank empty. For pusher configurations (engine in back) that would be tank full.

very nose heavy planes fly poorly, but tail heavy planes only fly once. You could surprise yourself one day, be ready to land with your usual 1/3rd tank left when someone plows their plane in on the runway, littering it with pieces. Now you've gotta stay up until the debris is cleared, and you really dont' want to be fighting a progressively more tail heavy plane! Especially with ppl on the runway, cleanin up for ya!

J