I have nothing against using one. It just isn't common practice in the industry. Everyone trusts their gauges to read reasonably accurately which most now are digital and those have even done away with the need for a seperate meter if you are going for perfection. Most everyone goes by the gauge and if the pump pulls down to 30 and holds there for long enough then it's good. Course there are guys I have seen from some companies not even pull a vacuum on a system and instead just flush refrigerant through it and they call it good...
You are doing it the way everyone should be doing it. I have been known to leave a vacuum pump running on a system for 2hours sometimes more. Longer runs don't hurt anything. Course the grocery stores we did we had to rent a diesel powered vacuum pump from a bulk refrigerant supplier then have them come charge it up.
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