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?