I use a plane tach, the min air one is nice but overpriced, & still req's a second person to run it.
I rubberband my tach to a screwdriver & stick it in the ground.
Hover over it, just at edge of rotor, too far in & it picks up the paddles too! Instantly recognisable, speed doubles.
If there are long shadows, put tach on sunward side of the bird.
I set up my Heli's this way and a guy showed up with a min air tach, speed was right on according to his tach too.
I plan on a GV-1 for my next upgrade too. Just got the GY401 & servo combo installed last week, Love it.

Russell