I take a different approach.
Place your calipers against the side of your cutting head, depth gauge pointing down, starting flush with the bottom edge of the black plastic (nearest to crumb tray).
Now, extend the depth gauge until it touches the crumb tray (you may need to move the head or calipers slightly to get the two small metal bits aligned to make contact). At this point, zero out your calipers.
Now, to measure the height of any object, you just put the digital calipers back up against the print head and extend the depth gauge once again. As long as your reading is negative, you just enter that (as a positive value) exact measurement as your material height. If you do get a positive value, then you need to add some material underneath whatever you are working on, and what you add should be at least as thick as the positive value showing on the calipers.
This works for even finding the material height of things when you ARE using the crumb tray, but forgot to measure in advance of putting it in the machine (I am frequently guilty of this one).
Of course you could do this with the calipers pointing the other direction, extending the head instead of the depth gauge. That would make it easier to hit the crumb tray regardless of positioning. Not sure why I do it upside down…