ALT1 and ALT2 calibration

I have a 60A plane power alt on the shunt as ALT1.
I have a 8A B&C SD-8 on the hall effect as ALT2.

I never calibrated these. The ALT1 has always seemed to display a reasonable value.
I always leave ALT2 in the OFF position, and it always reads -4A. Seems like an obvious calibration issue.

What's the best way to get an accurate calibration on ALT1, ALT2?

You can calibrate both using the SHIFT ADJUST function in Instrument Calibration. With the system online, but the engine off, set both to 0.