To my knowlege there is no tweaker to offset the reading. I think they are laser trimmed at the factory to hit a spec. Bench testing is difficult. It requires a signal generator and even then it's difficult to simulate the hostile voltages that a real ignition circuit presents.
Probably the only thing you can do to offset it is to remove the needle and put it back on again. But it'd be fairly certain it's 400 RPMS off at multiple values. It could be non linear in which case a needle R&R won't really fix it.
Speedometers are non-linear. Instead they are logarithmic. Tachometers are linear. If they are run by voltages, you'll find when calibrating them, that the full scale has an associated voltage, and if you cut the voltage in half, it will read half scale. This is course is from my own calibration experience in the Navy. Automobile tach's may differ, but I doubt it.
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