I don't have a microscope with a reticle available but I did want to be able to accurately measure the diameter of pinholes and the width of aperture slits. So . . . I shot a close-up of a graduated scale with a 35mm camera. I then measured the length of the scale on the negative between two widely spaced primary markings, say around 1 to 1 1/4 inches apart. [This in turn gave me the exact spacing between any two of the finest graduations, by the way.] The rest was simple. Cranked up an enlarger head to the top of the column. Put in the negative and focussed on a white sheet of paper. Marked the two measured divisions on the paper as reference marks. I then knew the magnification factor of the enlarger. Took out the negative and put the pinhole plate in the negative carrier. Marked the diameter of the light spot on the paper. Measured the diameter [I used a vernier caliper, but a good ruler would serve.] Multiplied by the factor. Voila! No enlarger? Use a slide projector. No slide projector? Cob-house one up using any convenient light source and any simple lens.