When I test cars after repairs (very special job on a hot summer afternoon), I drive them down a divided four lane for 3 miles at 50mph in high blow condition. This is enough time for the system to stabilize and I get to see how low can it go. I expect to see at least 40 degree off ambient temp to consider it "good" A/C.
Many 300Es (especially pre 90 models) that will meet this criteria will loose ten degrees off this measurement when I sit at the light at the end of the three miles. Many early 300Es with 134 conversions only make 30-35 degrees off ambient in this test (remember high blow).
Bosch Master, ASE Master, L1
33 years MB technician