i.e. a 10% increase in reaction speed for every 1 deg C increase. i use this rule of thumb for dev times on the plus and minus side of 20 degs C. However, I have generally found that when i have large changes, such as developing at 17 degs or at 25degs C, I reduce the amount of change somewhat. I generally shoot rollfilm shot in a wide variety of conditions, so add or subtract a little depending on my perception of the contrast of the scene. Never had real problems. In the summer my water temperature is about 23-25 degs and in winter 16-17, so I have to try to stabilise the room temperature for a little bit before i develop to bring water containers as close to 20 degs C as I can. A few degs C either way and the 10% rule gets me nice and close.