I was printing from one negative last night and had the 8x10 landscape print down pretty well using split filtration, then decided I wanted to crop in on it a bit more and orient it in portrait mode. I ended up raising the enlarger head probably from 10" to 15" or so. It was getting late and I didn't want to bother with another set of test strips, so I just ballparked it a bit and increased the time about 50%. The print came out OK, but definitely too light. I probably should have taken better notes before posting, but I do recall from a failed experiment in the past where I doubled the height of the head and I increased the time by 4x figuring the inverse-square law would work, but it just toasted the print. Has anyone done this experiment before and have a good factor for this sort of thing?