It would be cool if cameras could just figure this out on their own and overlay an accurate virtual reticle on the image. But they don’t, so when I’m in the field I just carry a small ruler around. Nothing fancy, just a 6-inch clear plastic ruler with black numbers printed on it, like from a kid’s math set. Will set you back about $1-2. I tweak it by adding yellow electric tape to the back so the numbers show up better (example: https://inaturalist.ca/observations/2829077).
A grid background can work great too. Any school supply place/site should have rolls of the stuff, in metric or imperial. One application: I cover the wall by my moth light with panels covered in grid paper (example: https://inaturalist.ca/observations/14028334). Another application: tape some to your clipboard or a smaller board for field use…
