Recovery Techniques
The target depends on two facts
For high-pressure appliances the required recovery level depends on when the appliance was built and how much refrigerant it holds. Pre-11/15/1993 appliances: recover to 0 psig if the charge is under 200 lb, or to 4 inches of mercury vacuum if 200 lb or more. Appliances built on or after 11/15/1993: 10 inches Hg vacuum under 200 lb, 15 inches Hg at 200 lb or more. Recovery equipment built before 11/15/1993 may use slightly less stringent levels. Questions love to swap 0 psig with a vacuum, or 10 with 15 inches.
Practice
Recover liquid first, then vapor; pulling liquid is much faster and protects the compressor. Use DOT-approved recovery cylinders, never fill past 80% of capacity, and weigh the cylinder on a scale. Never mix refrigerants in one cylinder - a mixed cylinder cannot be reclaimed and becomes hazardous waste. Recover from both the high and low sides, and do not forget the hoses and the recovery machine itself. Never vent refrigerant; recovery is required before opening a system.
Rule of thumb
Post-1993, under 200 lb: 10 inches Hg. Post-1993, 200 lb or more: 15 inches Hg. Remember that newer and bigger means a deeper vacuum, and the table falls into place.