The MAC is the Medium Aerodynamic Chord. You give the position of the CG in % of the MAC. Depending of its position you will need a different angle for the THS (or the trim tab in other aircrafts not the A320). You can find the required THS angles (1UP, 0, 2DOWN...) required for each CG between the thrust levers and the trim wheel (this is in the A320, but I suppose will be the same in other acfts.)
Is this what you're asking?