Hi Rich, I like the spread sheet idea, just not learned that yet.
Yes, can vouch for the tension too.
BTW, all tests were done with absolutely no machining load on the spindle.
When the heavy utility type v belt was on, I used 3 tensions. Light, med.(normal) and very tight, all of which produced different amounts of variations at the Mach reported rpm.
The looser settings were the best, but still unsatisfactory.
The tightest setting varied the most.
The relative output rpm isn't really an issue at the spindle as it is with an axis, just needs to be steady and consistent .... but, I see what you mean.
Thanks Rich,
Russ