Installing an Autometer 5" Tach 95 ESI

Where do I tap my tach wire into? I went into the coil pack and it is only reading half of my current rpm. I need help please.

For those of you who might want to know you follow the main wiring loom to the computer by the airbox. Look for the all white wire and tap into that. The tach is more true than the one in the dash nice upgrade! The rest of the wires go under the steering wheel.