Search this topic on YouTube and you'll find a DIY tach board calibration video. It gets you close by showing connections and zeroing technique which is very straightforward. However, I was in your same. situation after installing it. The tach was nowhere near working properly. So, while sitting in the car, I removed the clear speedo/tach cover and then firmly but very carefully removed the tach needle. I already knew what it's normal idle RPM was so I started the car and while it was idling I just reinstalled the tach needle at the proper idle RPM. Presto!
On an unrelated topic, I had noticed that the tach board got pretty warm during the bench procedure while zeroing the tach with my 12 volt power source. It obviously draws some current, making me wonder if this would help explain why I can only power my radio for about 25 minutes - say, while doing a necessary diagnostic/repair (i.e. replacing power door actuator) and with the radio playing in the key on (not "six" mode -before the battery dies. Something draws a lot of current to kill a new, fully charged battery and I wonder if the tach board isn't the culprit.
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