^ And, it's ALMOST correct.
To truly calibrate a frequency meter, which is what the tach is, you MUST use more than one calibration frequency. Simply using a 120Hz signal is not enough (because you are modifying the frequency response of the meter).
Sengk is very close to hitting it right, but, you have to remove the needle on almost every tach to get the "offset" correct. The pot allows you to adjust "gain" (needle swing) only.
So, to get it really accurate, you need high and a low frequencies like sengk is using with an aftermarket tach, and, you need to remove the needle and put it at the correct spot once the full needle swing is achieved.
I also found that using 15K, not 22k, results in greater reliability. (This resistor sets the input voltage range to the tach chip, which is actually a frequency to voltage converter.)