It's a while since I flew the 737 but I got about 5,000hrs in them in the 80s and 90s so let's see if I can remember............
I think the MAC scale on the trim indicator is only valid for Flaps 1 and 5 (I seem to remember you had to make an extra correction for a Flap 15 take-off).
This is why it is not to be used in the cruise, as the flaps are retracted and therefore the balance of CP/CG will be different. Also, the thrust will be much less than at take-off, further altering the balance of forces in the pitch plane.
Our loadsheets had a 'fuel index' which corrected the MACZFW to give the MACTOW for various fuel loads. We used this technique in the cruise to find our current MAC.
Hope this helps!