I set up a macro in UGS. Here’s the one that makes this work for me:
G20 G38.2 Z-.5 F2; G92 Z.5; G91; G0 Z0.25; G90
Breaking it down:
G20 = set system to inches
G38.2 Z-.5 F2; = move Z down to Z-0.5 at 2ipm until it touches the plate (it never reaches -0.5, of course)
G92 Z.5; = set the coordinate offset to 0.5", the thickness of my plate
G91; G0 Z0.25; = set to relative mode and raise Z by 0.25" for clearance
G90 = set system back to absolute mode