I photography, art, cooking and most other things...............nothing is written in stone! That's why most folks here do their own testing of materials. Kodak's data sheet is just a suggestion sheet, but a very strong suggestion sheet it is. It's also one of the very best starting places for trying Kodak products. Now, if you "mix & match", say using Kodak Xtol and a different film it's then a whole new ballgame. I use Xtol replenished and started by using Xtol 1:1 times, but those times are not where I ended up for my negatives, enlarger light source and paper. Close to 1:1 times, but not exactly. Still, the recommendations made by Kodak are the best place to start. In my opinion anyway. For whatever that's worth? JohnW