This activity requires accurate measurement of time intervals of a phenomenon that is difficult to track. As we have done in Physics 9A labs, we can mitigate the tracking problem by using video capture of the action, with a timer included in the frame, followed by stop-motion study of what has been recorded.
But even with this, there is something more we can do, since what is important is the percentage uncertainty. To review what we have determined previously about this, see the "Improving Time Measurements" section of Background Material for Physics 9A, Lab 3. As for determining uncertainties in these time measurements, the last sentence of the Background Material for Physics 9A, Lab 1 spells it out.