CaptainSandL,
Yours is the traditional answer to the question.
But if you produce a loadsheet which uses figures for an aircraft with no auxiliary fuel tank fitted, why is it necessary to use this correction?
Your loadsheet will give you a figure for the MACTOW CG which you may then correct for the CRZ using your knowledge of what happens to the CG as you burn fuel from the centre and wing tanks.
Personally I don't use the 4% correction as I fly a -300 with no auxiliary tank. I merely enter the appropriate CG using the trim wheel indication once in the cruise exactly as you suggest.
The 4% correction is almost always conservative as you observe which got me thinking....
I hazard a guess most people are flying around with an artificially "more forward" CG location than they need to be...