Dear Jack,
Thanks for the clarification.
Measure back from the original TA to a point on the main campus road. Establish a reference point and calculate how many counts (say “x” counts) you collected from the original TA.
Begin again at the reference point and ride x counts to your new TA. Ride back to the reference point, take a count, then ride to the original TA and take a count.
You will have measured everything twice.
You need not even calibrate the bike.