Because of this thread, I pulled out my tach while breaking in an OS75AX just now.

Wearing really good earmuffs (competition shooting muffs purchased for highpower rifle matches) I had no problem telling the difference between rich, lean, and when the lean sagged. The tach showed the difference of course.

For one to help you set a new engine would of course require that you know ahead of time what rpm to expect, and that usually won't happen. So for you to discover anything with a tach requires you either had previous experience with the engine/fuel/prop before and know the numbers you want (so all you discover is if the new engine is like the old one) or you discover what the rpm is when you identify the way the engine is running (rich/lean/sag) by ear. As someone has already mentioned.......