Throw money at it …usually quit a bit
Figure out the error and forget it as long as it moves I think your doing well ,you don’t want to be getting on it anyway
It should be easily calibrated by any of the several instrument repair guys and I would guess for about $200
I wouldn’t spend that to calibrate a tach to a tired generator. It’s cheaper and better in the long run to just buy a new generator.
But he said he thought his gen was only off a bit and said the tach was the one off by a good amount.
BTW, I bought a new gen from SNG as my tach was fine. But it is sitting in the passenger seat in its box along with the new Vreds all in a storage unit waiting until I have a new place for all 
I decided to take the easy way out and send my tach to Mike Eck. $100 and a length of wire from the coil to the tach and I have a very accurate tach. The generator is no longer part of the system.
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FWIW I can also set up a tach to trigger from the frequency output of the tach-generator rather than the voltage. That way the weakening of the magnets doesn’t have any effect, unless they are totally dead. The down-side is that the tach-gen is still being used, so drive-dog and retracting tang problems are still a possibility. Personally I think that the most trouble-free solution is to do what Jack did and leave the tach-gen in place for cosmetics and trigger the tach from the ignition. The original tach is not adjustable and can not be calibrated to compensate for weakened magnets.
Tachs reading low can be a problem. I had one tach customer who told me he was no hurry, since he had to repair the hole in the engine block where the connecting rod went through because of over-revving.
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Mike,
Does your conversion allow for adjustability of the tach?
Which is why I converted my tach to not depend on voltage but frequency. Then it didn’t matter when the volts went down. Worked a charm!
I calibrate the tachs before they leave here, and they are adjustable.
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My tach, which I believe you converted years ago, seems to read a pretty consistent 300 rpm low. I base this on three data points: the calculated rpm at a given speed (incorporating tire size data), the tach reading on my timing light, and the bluetooth app used with my new 123 distributor while driving the car. Is it as simple as a trimpot or similar, or is it more complex to adjust?
There’s a trimpot on the circuit board inside the tach, so it needs to be disassembled in order to adjust it. However, I don’t know how you would calibrate it unless you have a precision signal generator on your bench. There are also adjustable weights inside to adjust the movement’s linearity, and the calibration changes when you put the case back on. If you send it back to me I’ll make sure the electronics are working properly and I’ll calibrate it for you.