Personally, I could never understand why anyone would NEED a tach to tune an engine. Or for that matter, even USE a tach to tune an engine.

An engine is going to run its best owing to many different factors. Engine size, fuel type and quality, Prop size, ambient temperature and humidity, etc.

So why is looking at what RPM you engine is spinning going to do?

Just listen to it. When you lean it out, it will increase RPM, then you will get 3 or 4 clicks where the RPM stop increasing, then it starts to go sick as it gets too lean. Just back it off to where it first stopped increasing RPM. Now, at Wide Open Throttle (WOT) point the nose straight up. If the RPM drop, hold it level again and open the needle 2 or 3 clicks and point it up again.

(Note, anytime you do this, if the engine is at idle, bring it to WOT and let it come to full RPM for a few seconds before pointing it up - I have seen people pointing the plane up before the engine ever gets to full-bore and this is not the proper way to do it)

A tach is one of those tools that are great when you know what you're doing, but bad in the hands of someone who doesn't.

I once saw a kid burn up an engine because he was only getting 10,800 RPM and someone told him to "tach it" to 11,000 RPM.

Boos boy, getting a 4-stroke doesn't mean it will break if you crash. If the guys at your field are using them, I suggest you ask them how many of them have ruined a 4-stroke in a crash