Finding the Uncertainty Limit from a data set

[MaroonR]

Hello everyone, I've run into a little issue with physics homework. During a lab we recently had, we were required to measure the amount of time it took for a ball to drop from different heights (2m, 1.5m, etc) using a photogate. Now that I'm writing the lab report, I have to find the uncertainty limit in my measurement of time. Here's my problem: I have 16 data subsets. We tested 8 heights with two different masses, and each of those had 3 trials. I was told to use the standard deviation, but when I looked at my course syllabus it told me I needed to have at least 10 trials in order for the standard deviation to be accurate. So as for my question, is there another way to go about finding the uncertainty limit? Or, could I possibly be misusing the standard deviation when I use it for each subset (each height)?

Edit: My professor listed an appendix online that says I should be able to see the uncertainty from a graph. If I graph a time squared vs distance graph, how can I determine the error from that? I'll include my graph

 

[grey_earl]

To see the uncertainty from the graph, you could translate your graph (the line you draw), keeping the same slope, up and down until all your data points are inside those two lines. They then give you the uncertainty. However, this method doesn't have _any_ mathematical background; here it's usually only taught in schools since they don't teach statistics there.