As mentioned before, connecting to the dash harness will read slightly lower voltage. Insure all your grounds from the dash are in good shape; connecting directly to the battery will cause a slight current drain even when the key is off. A possibility may be to connect the voltmeter wire to the idiot light terminal (#4 on a mechanical voltage regulator, term #1 on a internally regulated alternator). This point has zero volts with key off and system voltage with key on, engine running.
Factory setups don't worry about precision readings since factory units are not really well calibrated anyway, and the gauge only gives a coarse reading. To accurately check charging voltage, use a good digital multimeter at the battery.
Regards, Georg
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