To find an accurate MAC I too would suggest ignoring minor cutouts, in the first estimate.
In addition though, make an accurate representation of your tailplane or foreplane in cardboard.
Once you have your estimated MAC, mark it on the cardboard replica and check that it balances about the midpoint of the MAC, in two directions.
I once did this for a complex double tapered, double swept wing shape and it balanced perfectly about the calculated MAC midpoint in two directions.
The calculated MAC is in red.
That gives you the 50% MAC position. Now calculate the 25% MAC position, and I would still put the pivot just ahead of that to avoid problems if the servo arm fails (or the servo itself). An all flying control surface pivoted too far aft will run to extreme travel if it gets disconnected.
Then mass balance on the pivot.