Well, that is a lengthy response if I go into all the details. Anyway.

absolute z=0 is the machine top (almost).
The machine base (bottom of the stock material) is at -40mm. This way I do not have to recalibrate machine zero every time and your first movement to zero is avoided as well as zero is now at the top if the machine, not the bottom of the stock.
The curve is let's say at -37mm.
I have the setting so that the material top/bottom is relative to the curve. Hence a material bottom in the dialog is the aforesaid of -3 resulting at -37-3=-40. In other words, I cut down to the very bottom of the material.