Measurements from the material bottom

I have a project that requires an accurate base / mounting plate. For a long time, I was using HDPE white sheets 8 x 12 x 0.25 But these are no longer available from Inventables (or anyone else I could find), so I recently switched to Expanded PVC.
These base plates are an ongoing requirement, as there are ongoing sales. Expanded PVC thickness varies from sheet to sheet and also across a sheet. I see thickness differences of up to 0.025" with the thickness being undersized.

Because it is a base plate, the important measurements are referenced to the bottom rather than to the top of the material. So I needed to come up with a way to compensate. Here is my solution, perhaps this will be helpful to someone else.

I first measure the thickness on the corner that will be under the lower left corner on my Carvey. For example, the most recent sheet measured 0.231 So I “lied” to Easel by telling it that thickness was 0.250. Then when I put the sheet on the Carvey, I place a feeler guage of 0.019 on top of the sheet. Then proceeded to carve as if it was a uniform thickness of 0.250 Most of the carving is all the way through, so while there is a bit of wasted time carving air, it all works out. I have a few holes that require an accurate distance from the sheet bottom. For these I cut a shallow depression in the top side, and then use a standoff. This method results is a uniform height above the bottom of the material regardless if the actual material thickness.

You could just set the wasteboard as Z-0.250.