Your understanding is correct, as is your concern about relocating it (at least relative to the other identical test cut locations). I hadn’t resolved that issue yet, although I have a feeling there may be a way to do so, I will keep noodling on it.
I have a hunch that a strong (resilient) set of mylar template and some ingenuity can help us go a long way, especially if we can find a way to index them to a known surface for repeatability.
It strikes me that mylar is likely roll cut rather than sheet cut, which means it could be feasible to to have a calibration set cut that is in two pieces, 1) 4" x 96" and 2) 4" x 48". Just whiteboarding here but if you did that you could index them to the corners of a stock sheet (or even a cut down sheet, and adjust for the reduced size in GC as is already done) and then do the calibration from the known locations on those.
I’m thinking have them meet in the top left corner (indexed to the factory edge of the MDF, maybe specify 1/2" or 3/8" MDF to make it easier and more affordable to acquire and move?) and do a set of readings, move the 48" mylar piece to the middle or far side and do another reading, drop the 96" piece to the middle or bottom and read again, then move the 48" back to the left with the 96" still across the bottom and do a final reading. That would remove the need to measure by hand and give your optical sensor shapes (or other markings, doesn’t have to be cut through on the mylar).
I don’t know about the expansion/contraction risk of mylar or an equivalent, but assuming that isn’t an issue, it could roll easily to ship in the kit box. Perhaps in a cut down poster tube to protect it from damage.
Certainly having a known, consistent, laser cut set of references seems like it would go a long way to improving the ease of calibration and this is one way to do it in a light, small, cheap to ship way that can be extended to include optical calibration support (without requiring the user to use it, in the spirit of different price points or expandability).
@madgrizzle My “edit” turned in to practically another post, so tagging you again so you don’t miss it.