I usually use a tach when setting four strokes and gas engines. I also use a tach on two stroke glow engines to check idle speeds. When you lean the high speed needle the RPM increases until it peaks. Then you richen it out a little. It's easy without a tach when tuning a two stroke glow engine because you can hear the pitch of the engine increase and decrease. All engines tune that way, but it can be more difficult to hear minute pitch differences in a four stroke or gas engine because the pitch range is lower. This is where the tach comes in handy since the numbers don't lie.