I am stingy on tachs. Now that I've said that, my first tach was a kit built one out of 'old school' ACE radio control, Higginsville, Mo. for 30 bucks and you adjusted the sensitivity by pointing it to a light bulb, remember those or a fluorescent tube and with that it would count the AC cycles of the light and thus you can adjust the tach to 'zero'. Still works ok.
Then I went to a TNC, wanting to buy 1 more time, this was well before the 'mini tachs' you see now. Spent $100 dollars and at the time, besides the 2 and 3 blade choice they normally give you, I was building a P-51 giant scale at the time and was incorporating a 4 bladed prop, in doing so, I asked 'TNC' which was the original person that had designed the tach if he could incorporate an '4 blade option'. He did, with a small micro slide switch that is separate on the side of tach and the option works like a jewel. Bullet proof tach it is. And I really like that you can 'point' the tach a distance away and not be threatened by the whirling prop.
Mike