As Floyd mentioned, calibrating at line current frequency only means the reading is correct at THAT freq/RPM, but it is a start...
Most of the available tachs I've seen CAN be opened (Haven't had a close look at that red one, though.) There are trim pots somewhere on the circuit board. They can be calibrated on a fresh battery with the case open, but there may the reading may shift when the box is closed back up. Watch for that and accommodate it.
I've heard that LCD screens are sensitive to direct sunlight; hadn't heard about high temperature affecting them. Thanks!
As to direct sunlight, I even turn my Casio LCD wrist watch under my wrist when driving, or out in bright Arizona sun for extended time. It's now about 4 years old, and been through a few battery replacements, but still has a clean, bright display... Hasn't burned black yet, either...
Still have a Vibra-Tak from the 1950's, and the collet is still nice and tight. Never did stick the pin through a prop - yet, anyway. It did "work" as consistently as the operator let it, and in the 1950's there were no electronic alternatives. There WERE revolution counters, where you pushed a padded shaft end onto the spinner or prop nut and timed the count to get the RPM (RPM = counter reading X 4 for a 15 second timing, e.g.). Never interested in THAT...