I have a design that is 11.8" tall and I am trying to print it on 12x12" 3mm ply. When I go to frame the print, the marker goes off the end of the 12" board by an inch or so. It is ligned up right bat the lower edge of the board. I put a very light box around the object to see where the actual cut would be and it’s too big also. Other objects I have cut seem to be close to the right size.
Lay out and cut a 10 inch square, then measure it accurately. If it’s not the exactly 10 inches on a side, use the actual measurements to recalibrate the step/mm configuration for the X and Y axes.
The error scales with object size: an inch-per-foot error will be barely noticeable on a 1 inch object.
It’s worth checking, just to eliminate one possible cause.