I've made a ruler where the basic unit of measurement is the standard key space (19.05mm/0.75"). I made 15 of them in the first batch. Once they sell out they're gone, unless there's more demand than I expect.
You can use this to measure keys. Big keys, small keys, multiple keys even. It's 16u wide (12") which is enough to measure a 65% board.
They're not perfect, but they're perfectly functional. The Clueboard logo may not be recognizable on your ruler. There are scorch marks on the back because there was no backing. You can sand them with a fine grit sandpaper to remove that, or leave it.
To purchase PM me your email. Please let me know if you are outside the US. They are $5 each plus shipping from 95066.