Stack ST400 tacho not reading correctly on 13B rotary

[JAFA851]

As the title states, my car has a Stack ST400 tacho hooked up to the dual system electronic ignition. This is how I bought the car ( NA, carburettor, dual system electronic ignition ). The back of the tacho has a plate with the serial number, and it also says "6 cyl". The tacho works, but indicates about 7500 when it's actually 10000+. I don't know if it's accurate at the low end or not. But how do I get it to read accurately, or is it not the correct version for rotary engine?

I have seen a video of the car being dyno'd before I bought it, and it had a different Stack tacho in it, and that worked and indicated correctly. Don't know why it was changed, but it was, and now I need to get it sorted out.