i am taking this info from a physical manual loadsheet
suggest that you get a copy of the TCDS, the AFM and reverse engineer the trimsheet to figure out the actual numbers .. quite easy to do once you get into it .. and then set up a normal calculation for the PC. Trying to make a trimsheet directly into something else is a recipe for ulcers and frustration.
the datum is 6.3825 meters forward of the nose
if you have such a datum for the trimsheet, forget the exercise ... the trimsheet will be a dog's breakfast so far as (in)accuracy is concerned. More likely, the basic OEM FS datum is forward of the nose .. but the trimsheet uses a more appropriate trim datum (located somewhere in the envelope) .. you can tell if this is the case by the shape of the envelope on the trimsheet .. if it's long and skinny and slopes from lower left to upper right .. then the datum is somewhere out the front (and whoever designed the trimsheet really doesn't know what he was doing) ... if it's sort of squarish and boxy .. the (trim) datum is in the envelope. I've not played with Airbus but I could not imagine that anyone in his/her right mind would have designed a trimsheet using the FS datum.
MAC overlay
the only reason for you to worry about MAC is if your operator prefers to use MAC to describe CG and (as would be the case) the loaded CG/flap defines stab setting for takeoff. If the present trimsheet has a CG (or MAC) overlay, you can reverse engineer this in a doddle as part of the larger exercise.
Why don't you scan and email a copy of your trimsheet or post it somewhere and we can offer specific advice ?
(Logarithms don't come into it at any stage ...)