Your beast is lightweight; if it's side-by-side, don't forget to take into account the very significant lateral CG shift between one- and two-crew. The change in AUM will probably be of the order of 15% between one and two crew.
If it's tandem... Well, I'm sure you know all this, Genghis.
Once had a serious blockage (mental) during a RW, ground observed, project with very inconsistent results between successive reciprocal runs along the RW during the same sortie. Turned out it was the enormous shift in lateral CG caused by the on-board FTE changing from side to side so that he could be on the same side as the ground observer. "Ball-in-the-middle" meant nothing to the static ports that were in different flow each time the FTE shifted. He was quite a normal sized chap really! Got him to sit in the middle and the trial continued...