ditto what Dick said - the human ear is one of the best tuning tools you can get, if you have a good ear. I hate hearing a 2 stroke twin that one engine is about 2 clicks leaner than the other, I mention it to the other guys around me, and none of them can hear the dissonance. I go and tweak the engines for the guy and then everyone says - ahh sounds better now.

I found that it is best to not have the plane pointed where you are aiming your tach into the sun. Also - make sure if you are under your carport, or in an enclosed shop, that you don't have any overhead incandescents or flourecents turned on. I've had issues at times with the carport lights throwing my tach reading off.

The instructions in my Hobbico tach stated if your readings are fluctuating, then you need to turn the model 90 degrees - never had to try it, but it's something to remember.