The majority of time spent for anyone is in training not in competition and if you are not able to rely on timing regarding a 10, 20 or 30 meter then what guideline might one have for the basis of your overall training ? Ideally you need the same person around to time you and over time this person needs to be movtivated to learn consistency of how to time accurately . I don’t think we want to dismiss timing for short sprints but we do want to understand the magin for error and how over time our timing translates into our longer runs and ultimately competition.