Put the tripods at the lowest setting, on the line and go from when they break it with the shin. Not exactly from first movement, but we do it the same way every time. Probably a 0.10 to 0.15 off of using it off first hand movement. If anyone bitches about it, we show them their 10 to 20 yard segment which you can compare with anyone else using the Speed Trap Timers. We have a good number of guys who can run a good 0 to 10 yard, but it seems the men are separated from the boys on the 10 to 20 yard segment.
On average our decent guys are running under 4.60 seconds. We had two guys last year under 4.40 and four guys under 4.50. I believe my timing method is balanced off by the fact that it is usually still bloody cold when we test in late March.