The compound gauges of a refrigerant manifold are good for gross measurements but are not accurate enough to determine exact vacuum levels. The last inch of vacuum (29 to 29.9" Hg) represents the difference between 23,000 and 250 microns, which the gauge face is simply incapable of reporting.
Without an accurate means of measurement, determining the correct evacuation time becomes guesswork; you waste time by running the pump long after evacuation is complete or, if not long enough, you risk not removing all the moisture.
Does anyone routinely use a thermistor or electronic vacuum gauge in conjunction with their manifold during system evacuations? If so, is there a preferred gauge manufacturer?