The little Hanger 9 tach reads from a couple of feet and if you setup an accurate aim -and see a solid reading --the reading is as accurate as any expensive optical- tach
I have checked readings compared against the direct ignition readout on various engines -- Evolution and ZDZ - the differences are trifling.
All of my engines have direct ignition readouts . (ZDZ and Evolution).
It takes out all of the guesswork -on the ground or in the air.