Alternator Gauge

I have a 74 duster with a slant six.
When i'm driving, the needle in the alternator gauge stays about in the middle, but when im braking and at idle speed, the needle dips to the left quite a bit. I've installed a new voltage regulator and that did not fix it. is this caused by the idle speed? and if so, which screw do I need to tighten or loosen on my carburetor?

Thanks