Realized that I ought to elaborate a bit more. Am familiar with using rulers to set up vision inspection camera systems when the actual parts aren't available to use. Have found that satin finish rulers are best. The graduations I prefer are engraved and then black filled with a low lustre pigment. Have also used the same rulers to adjust micro focus. Using live view and manual focus, I focus on one specific measurement line or a target aligned with the ruler depending on what I am setting up. Take a picture. Turn on auto focus and compare the results of a few pictures. This is faster to do when tethered as it is also possible to adjust the focus from the PC. If the results are not close enough to keep me happy, I also know which way to adjust and just record the amount of adjustment required for adjusting in the camera settings. One other problem I experienced once when I had the ruler further away, was a pixelation issue which caused grief in assessing the results. The ruler was too close to horizontal which caused the relative width of the graduations to drop below one pixel width on the sensor. The separation of the markings was also very small in the images. Changing the ruler angle increased the apparent width of the markings in the images, while at the same time the separation increased for better resolution of the focos. It might be a good idea for you to back calculate the pixel resolution in millimetres using your sensor specs along with the image circle dimensions for the lens combined with the optical angle of view. Pixel peeping on one of the markings in an image would also give you a different path to the same information with no math involved.