Steve;

I think the fine tuning is for the alternator set up. Some tachs can use the pulses from the alternator to calculate the RPM. For example this method can be used on diesels where there are no ignition pulses coming from distributor/coil. The pot let's you tweak the signal to coordinate the pulses to crankshaft RPM because the alternator is not generally turning at crankshaft speed. You should be good to go by hooking up to the ignition. There usually is a diagram for both conventional and electronic ignition systems. Good Luck

SimS