I am looking for a reliable way to measure the aperture marks on my lens with the eventual goal of adding f/32 to an aperture whose markings end at f/22. I tried placing the camera at a fixed distance from a bright 500W light. I put my Sekonic light meter at the film plane and opened the shutter on bulb to take a reading. The results weren't quite consistent given the apertures as marked so I'm a little hesitant to use the readings to mark the f/32 point. Is there a better way to do this? Perhaps the light fall-off from vignetting is affecting the reading? The light meter sensor isn't quite in the center of the film plane. Thanks!