After quite a lot of experimenting and then cutting parts using Estlcam and setting the z-axis origin to the Machine bed with the aid of my z-probe (Setup/Basic settings…) I still have an almost perfectly unblemished spoilboard. If you set the z-axis origin to the Machine bed, Estlcam does the rest. The only difference is Estlcam asks you how thick your stock is.
It is vital, however, that you surface your spoilboard very well (I did so three times, just to make sure, taking off .5mm each time).
It certainly beats having to measure your stock in various places, and then add a “safety factor over-cut depth” just to make certain you cut all the way through your stock.