for me, I dev. and fix , , etc an unexposed sheet of film, to determine base + fog. then placing that neg. in the neg. carrier, raising the enlarging head to the height of any particular degree of enlargements ( usually for me its the same as always, full neg on 8x10 or 11x14). I make test strips for each paper grade to find maximun blackness. at each intervel moves i note the time, when no longer I can distinquish any niticable difference after fix and dry down. I Now have a starting point for time. However it is just a starting time. Ie. One neg may have been shot( exposed) higher or lower on the curve! some negs were over developed, some under dev. and, of course, some "just right"!! different grades, different choice of developers, different dilutions, differing temp and of course time. all inter relate. its all very fluid. I can not say, "that I have a standard time" for exposure of paper, even though I use the same film, and developer combo, all the time.