I think ger21 means the surface will be gone after the first cut.
I use a surface gauge and set my tools from it whenever my parts surface is removed.
You can also use any known highth block, the vice, or the table. Block should be machined.
Set the block next to your part and measure the difference between the top of your part and the block.
Record the difference and adjust the Z offset.
Double check your math and what you type in the control.
Write everything down in case of intruptions.
Scott
Some of my best finds were in the trash....