Sony E/FE focus testing screens

I am currently working on some basic designs with simple optical elements in 3D printed housings. At the moment my optical elements mount with M42 threads onto simple helicoids that I adapt to Sony cameras. As I get towards finished designs I'd like to know the lens focal length with more accuracy. The best way is to focus the lens at infinity using the camera itself and then try to measure the total length to the focal plane marker (or just to the flange and then add 18mm). But that is fiddly and involves constant lens swaps or scrabbling around the camera with callipers or a steel ruler. Removing the helicoid to measure it invariably adjusts the helicoid! So to speed up some of the initial measurements I have printed a simple focussing screen mount on a Sony lens cap, using JP 's remix of LXO 's rear lens cap (largely because the former was already centred around the axis; I don't pretend to be an expert!) This simple design, while not precisely accurate, makes it easier and a lot quicker to use callipers to measure the total length from the front of a mount to the focus plane. As I have an idea how far the lens centre is from the front of the mount, I should be able to calculate the approximate focal length. And it will certainly make it easier to figure out how much tube to add to a design to have it start focus at infinity. The “full” version should have an 18mm flange to screen distance to match the Sony mount flange-to-sensor distance. In the “shallow” version I have cut the cap down to the thinnest sensible size to make it easier to measure short focal lengths or when using a bellows. Use any thin translucent material for the screen – just tape it across, making sure you can get the callipers across without too much added thickness. For now I have used some diffusion material from an old Lee Filters gel sample pack, which has negligible texture. To find a practical infinity focus, point the lens at a very distant tree or point on the horizon, and adjust. In my final designs I am likely to err slightly short of the resulting measurement, because it means that any issues with dimensional accuracy are more likely to leave a lens that can still focus on infinity.