Same here, I own a tach, but I only bought it to show the RPM in a review I once did. Haven't touched it since.

Plus, you really CAN'T go by RPM! The engine is going to swing different props at different RPM. So what may be a perfect setting with one prop, may be too rich or lean with another.

Just lean it out until you hear it reach it's peak, then back it off to where it first hit it's peak, or one or two clicks richer. Point the nose up at full throttle. If it goes sick make it a little richer.

If you are tweaking it, and you get it too lean, it's not going to hurt anything for as short a time as you're tuning it. The worst that can happen is if you get it way too lean, it may back-fire and throw the prop (A good reason to keep yourself and others away from the front of any running engine). But you'll usually hear it go sick long before that happens