Question about calibrating a root 4

I have a root 4 CNC, I purchased used from someone I know. I will start with I cut things on this that are dimensionally correct. For some reason, it seems to get lost on bigger files.

If it’s a quick 5-10 min cut, there isn’t any issues. Longer cuts with a bunch of different positions it gets lost and does do the dimensions correctly.

What is the best way to calibrate the movements? If a dxf is in cm and I’m doing inches with grbl, it should do the correct move?

Some are correct and some are not. I’m using FluidNC to run the gcode. Is there something better for this?

Thanks for any help on what to check. I am squaring the machine and it is staying in square. For example, it seems to be off on the Y plane and I am running a dual axis Y.

Cut area is roughly 33” x 68”