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I would be measuring the frequency off the generator output. This will take into account all variances like pulley wear and give you an absolute speed/ frequency. Having a Multimeter fetish I know there are a load of meters available cheap as chips from China with frequency which are all amazingly accurate. You should be able to get one for $10. The panel frequency meters aren't expensive either. Meters are better, simpler and more accurate than anything else I can think of.