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Sure, that's a measurement of frames dropped to some extent. Depending on what you're trying to measure that could be a useful measure or not. If you're that close it's likely you got all the messages during the recording but had some missed at startup and shutdown of the recording.

Depending on your setup the messages arriving from different sources might start and end at different times as it takes some time to establish connections at startup, and exactly how the system shutdown there might also be extra time at the end from other topics too.

I'll note that you're using a rounded length of the bag file, the more precise length of your bag listed is 699.81s which is several more frames equivalent. You could also look at the actual timestamps of the first and last logged camera images to detect losses in the stream.

If you want to be more accurate you can write a script to look at the timestamps and find out potentially not just how many were lost but which frames were lost if any during the stream.

You need to evaluate whether the metric you're using is valid for whatever use case you're planning to use the metric for.