It's easy to fall into a Rube Goldberg approach to simple things, but I was curious about using negative height, so I made some measurements -- Exposure Value versus negative height, or EV versus lens height. Negative height is easier to measure on my enlarger, but it is not ideal as it does not follow the inverse square law (ISL). A quadratic equations fits it nearly perfectly, but it's not as easy to use as the ISL, and probably unique to each enlarger setup. Lens height follows the inverse square law very closely -- the main error in my measurements is EV value. I may try and improve these measurements when I have time, but the error is mostly about 1/6 stop. Of course, the light coming through the lens at the iris could be assumed to be a point source, so there should not be much concern about using the inverse square law, or enlarger head characteristics above the point source. Getting a reasonable estimate of a new time for a new paper size (say 8 by 10 to 16 by 20) via the ISL, and using this estimated time on a small paper (like 5 by 7) at the bigger enlargement, is usually faster for me than starting over with a test strip. The narrow strips are usually not wide enough for me to judge the final outcome.