Yeah, I agree that that is the realistic way to do it.  But the problem is, we have no way under the Captain Sim scheme to arrive at trip fuel, since we are told to use FSX in lieu of a fuel planner with ACE.  So I am trying to work within these limits.  Without an accurate trip fuel calculation, figuring how many pounds you're going to burn with 20 minutes on the APU or something seems like a waste of time to me.  I would rather have a safe, reasonable estimate.  I think I can probably do that off the FSX value.