Would G0 Z0 work after zeroing the Z-Axis on the first piece of workpiece?

G90 is absolute mode (will move to a certain position no matter where it starts) and G91 is incremental mode (moves a specified amount in a specified direction) so...
G0 G91 Z-.85 will move the z down .850". If G91 is still active G0 Z0 will do nothing (it's telling it to move 0") If G90 is active G0 Z0 will move the z to the position of z0.
Normally programming is done in absolute mode with incremental used for more specialized things

HTH

Mark