I would recommend using a counter task, specifically a finite buffered period measurement with at least 3 samples. From your array of periods, you'll want to ignore index 0, then sum indices 1+2 for your oscillation period.
Index 0 will be the arbitrary time from when you start the task until the first active photogate edge is seen. In many apps, it's meaningless and should be ignored.
(Note: The initial meaningless interval is measured on older device designs like your M-series board. A newer X-series device would suppress this initial interval so then you'd simply sum indices 0+1. This change was made as a non-negotiable "favor" so the behavior is present *all* of the time, but it turns out it's only actually helpful *most* of the time.)
-Kevin P
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