The way your using your plate as a z home switch should work. When you do this and the number is some number other than the +0.230 you have set for your home off distance, are you looking at the machine coord. DRO or the work Coord. Dros. I'm assuming work Coords becasue when you do a ref all home the machine coord. dros of x and y should zero out by them selves if you do not have home switches enabled in ports and pins for the x and/or y axis. When you say
I then set X and Y to 0.
I think you are zeroing your work coord. DROS by clicking the zero x and zero y buttons beside their respective dros. This will not zero the machine coor. dros although they should be because you just referenced the machine (if you haven't moved it). Run through your routine and before you jog press the MAchine Coord. button below the dros and see if the z number is what you set for your z home off distance.
Brett