Tachometer calibration

[425HP409]

I hooked up 3 tachs to a car with a digital timing light for a reference tach and 2 calibrated perfectly throughout the rpm range with the screw in the back. The 3rd would calibrate at idle, but read 2000 when it should have been 2500. I have 2 more to check tomorrow. ps: The test car had HEI and one of the tachs had previously been in my truck with point ignition. It read the same before calibration with both ignition sources. The tack reads ignition pulses, and doesn't care if it is HEI or Points.