Machine off
Machine on
$H homing cycle
Zero to work X,Y bottom left using manual position 3-2-1 or other homing jig
G28.1 to save this position as offset from $H final position
Zero to work surface using paper, eye, or other homing jig (triquetra?)
run 3d roughing cut g-code
Raise z and change bit
$H homing cycle
G91 G28 X0 Y0
This returns to same X, Y when you ran G28
Zero to work surface using paper, eye, or other homing jig
Run 3d finish g-code
Lather, rinse and repeat