(10/23/'20 Edit - I added a double-sided version here.)
I present for your consideration - the Focus Height Ruler (cubic Smoots per Wiffle version, I might do a metric one later…)
FILE IS HERE -> focus height ruler.svg.zip (6.9 KB)
(Three steps - Engrave the numbers, Score the lines, & Cut)
As focus height should be entered above where the crumb tray bed should be, this can be handy for determining that measurement when the bed is not there - or just to check your material thickness. It uses the base of the head as a reference point, so as you can see, when placed directly on the bed, it measures “0”. Note - this is NOT 0.5" below the head (it’s actually around 1.3785" on my machine.)
As you might expect, it measures 0.125 for 1/8" acrylic…
… and unsurprisingly, 0.500 for a 1/2" lump:
So when you remove the tray and prop up your (<2" >0.5") target for printing, you can use this to quickly get the focus height. [☞]
Of course, if the ruler doesn’t fit, or is too short, you need to adjust how you are propping up your material, because the surface is outside of the supported focal range.
[☞] There are many other solutions to this problem. This is just how I’ve been doing it, and figured I might as well share.