How to use Z-axis offset when cutting materials?

for 3mm material, the difference will be barely visible, unless you use a very short focal length.

with a 1.5" focal, and 6mm material, it begins to be useful to lower the head by 2mm. in my experience, it is not the best to target the exact middle of the material.

with a 2,5" focal, this makes no difference offsetting the head for such a small distance.