Quick Engrave Cutting Too Deep

I also ran into this problem.

Solution: level the board with a 20mm end mill. This will set your height uniformly across the board.

After that, my Z values were perfect.

Try using another part of the board and check the results. Another way would be setting your end mill to Z0 and let it run from Y0 to Y800 or whatever size is yours. If your board is not leveled, it will cut it at some point.

Thanks, unfortunately I won't be able to face off the surface as I'm hoping to engrave on a gift box.
I doubt the workpiece is deflecting that much but with the original stepcraft clamping it might be
possible. I think it is time to invest in a dial indicator and new worktop with a better clamping system.
In the mean time I might break up my engraving into multiple toolpath sections and zero the Z above each
section to minimise the impact of any deflection.