brooke,
You can use your tach to set your engine, but you don't start out with it being the deciding factor. Matter of fact, you merely use it to record the "correct" setting and then later to repeat that setting.

Our engines are dependent on the correct fuel/air ratio being set for them. And that's what we do and do the initial setting with all the tests and twiddles that bluestratos and chuck and a few others have mentioned. You really are discovering what needle setting works for your engine with the prop that is on the airplane that day. THEN.....

After you've got your good needle setting, tach it. And keep that in mind for next time.

Next time out, you use the tach to help repeat that good needle setting.

If you don't work out the optimum fuel/air needle setting (with all those tricks just mentioned) your engine will probably never run dependably nor with much power. It'll probably always give you fits. You gotta set it by the way it runs, not by any rpm numbers. And then you can record that rpm for that make and diameter and pitch of prop for later use.

Tachs are BIGTIME helpful for 4strokes. They're rather "flat" running engines and will sound about the same when a bit rich or when too lean. But they won't be turning exactly the same rpm, and a tach will tell you what you can't hear. And they're helpful with 2cycle engines too, just not as important.

They're a secondary tool, not a primary. But the guys who know how and when to use them are miles ahead of the guys who don't.