When an airplane is re-weighed typically after a heavy check, a new MAC is calculated.
[As you have done, I use MAC loosely to mean %MAC.]
Not a common approach but no reason why not and, depending on the trimsheet design, it may be necessary for the entry argument.
This latter consideration is for those trimsheets where the IU is not specified explicitly but is determined implicity by use of a graphical entry using empty weight and MAC (or CG) to generate IU. In general, there is no advantage in doing things this way but some trimsheet designers like it. One such which comes to mind is the original OEM Nomad trimsheet.
When calculating the IU for a more conventional trimsheet system, the general equation for the entry argument is
IU = C1 + W * (CG - trim datum)/C2
where
C1 is a simple shift to get rid of those pesky negative IU numbers.
If C1 is not used then the zero IU is at the trim datum CG position. Trimsheet datum can be picked out quite easily if the sheet has an MAC grid and is the MAC whose line is vertical.
If a trim datum is not used the standard datum usually is way out front with the result that the envelope slopes towards the right with an adverse effect on trimsheet accuracy.
W is the empty weight
CG is the calculated empty weight CG
trim datum is a reference CG position chosen by the trimsheet designer to improve usability and accuracy in service.
Typically, the trim datum will be somewhere in the CG envelope with the specific position chosen to suit the designer's preferences. The trim datum changes the effective datum for calculations; the end result is to change the appearance of the CG envelope as drawn in the trimsheet by making it more vertical. Each designer, naturally enough, has the view that his/her trim datum philosophy is superior to all others
C2 is the usual moment to IU factor
If you have the MAC, rather than the CG, it is just a matter of swapping out the CG for CG expressed in terms of MAC in the equation. Just complicates things for no real benefit unless you set it up on a PC.
He probably means the location of the empty CG has been adjusted
That is a near guarantee with a reweigh. However, one checks that the running CG position is sensibly close to the new weigh figure.
Addendum: This
old thread has some details on the trimsheet side of things.