We time it from the start of the take off run. Gives a level of consistency. There is a timer built into the strip board; when one departure is stamped as rolling, it starts counting up to three or four minutes, depending on the WT category.
I guess it probably should be timed from the respective rotation points, but there's not that much variation in time type to type, from start of TO run to rotate...maybe a max of 30 seconds for, say, a lighty behind a heavy. (Could be more if it's something like a Cub following an IL76).