I use an enlarging meter, the Jobo Comparator II to be exact. It is sensitive to the safelight, so you don't want the safelight on while making measurements. I also find it much easier to compose and focus with the safe lights off. I use a blank sheet of paper with grid marks drawn on it for composing. You'd be surprised at how much easier it is to get the image lined up properly that way. It is difficult to see the black blades of the easel with only the projected light hitting them, but that's also rectified as well. I've simply covered the edge with a piece of masking tape. It's not white like the paper, so you can see the difference; and you always know exactly where it is in relation to the image. Rocket science it ain't.