I don't think that the dash cluster gauge is percise.....what I'm saying is that I doubt that it can be used as a diagnostic tool.
To determine the system's actual performance or to actually see what the various voltage outputs are in various situations, you need to either use a volt meter hooked up at the alternator's output terminal or maybe at the battery terminals or hook up a scanner and scroll thru the readouts till the alternator's output is displayed. I don't think that the gauge was meant to be any more than a warning that there might be an issue developing with the charging system...ya know the gauge was not meant to be used in place of a volt meter or a scanner.
Tom