Calculating from a video is definitely accurate enough. You can figure out how many pedal revolutions are occuring during a set distance. 1 tooth on the chainring is not such a noticeable difference. Yes it can be important when you want to really fine tune something (like in a team sprint), but no one "accidentally" lost a race because their chainring was off by a tooth. If the rear cog is off by a tooth, then that's another story.
Sort of related funny story:
When I was 16, I used to race the Kilo in 49x15. I had tried 50x15 in training and decided to step it up for this race. When I was warming up, an older team mate offered me his disc to use. He thought he had put a 15 tooth cog on, but it was a 14. Rolling around the infield, I didn't notice the gear was that huge. Apparently I looked like Greame Obree rolling around the track in a 50x14 during my Kilo. I turned out my usual Kilo time despite the gearing.