During our indoor fly zone work, we use the Accusplit 740MXT which can be configured to record stride rate.
For longer durations (outdoors), we use a modified Fitsense system. We utilize an acceleration patterning process similar to that of Frye, Anderson, etc. It is tailored to the athletes’ anthropometric calcs.
Do a 30m fly with a 30m run in. The number of times a step is taken with a particular leg between 30m and 60m is ascertained using two counters (one per leg). Add steps for each leg and divide total by flying 30m dash time.
ex. Right leg + left leg = total strides
total strides / time = stride frequency
You can even set up 1m intervals for your analysis by buying some cheap tennis balls and cutting them in half and placing them at 1m intervals and with cones at each 10m interval.
As for when you take the measurement, just do a 60m sprint from the blocks. It’s going to tell you far more than trying to analyse a drill and would be a good place to start looking for areas to improve.