I have only been making my own enlargements ('print' just doesn't feel like the right word?) for a couple of months, I am almost completely self taught from books and advice given here on APUG. At the moment I am enlarging mostly onto 5"x"7, I have made some 8"x10". I now have several prints I would like larger, not much, maybe up to 6.5"x8.5". Is there a ratio I could use to calculate the new exposure time? I always catalogue print exposure time (including dodge and burn details), enlarger head height, f stop and filtration, so I have a point to work from. The reason I ask is I do not want to use too much paper. I don't mind using a couple of sheets cut into test strips, but if there is a way to get close to the correct exposure time without several tests, I would be grateful to hear it! Thanks in advance, Andy.