A couple of things
1. 10m testing times are useful when applied to other testing criteria such as 30m, 60m, SLJ, STJ, etc… Ask yourself why are you measuring 10m times and its application to training and improvements in training and performance. Obviously in track and field it’s application is a sprint start and more specifically initial acceleration. If someone’s 10m time drops, but his 30m and 60m test scores don’t then it’s likely his 100m or 200m times won’t be significantly different than before. Meaning you are likely, but not certainly spending way too much time in the blocks or first 4 steps of acceleration in training. A 10m time with corresponding 5m segment times in a 30m test is a better evaluation of initial acceleration anyways (10-15-20-25-30) and a 30m time has a better correlation to predicting 100m times.
2. It’s best measured from the initial movement to initiate action (hand pressure, block pressure, etc..) as Mike stated, however keep the mechanism to start timing the same.