In the first place some of these testers are not using the Standard FSM criteria...
Windows Down,
Blower on LOW,
and enough air across the condensor..... very hard to do without special equipment... you need serious forced air into the grill of the car....

absent the sufficient air across the condensor.... Which is the Bottle Neck for ALL AC systems..... one can not trust the diagnosing completely.....
for instance... in the Mercedes AC FSM.... it warns that something like insufficient air across the condensor can cause Air Bubbles to be present in the Site Gauge EVEN though the system is fully charged..... and even one oz overcharged can wear out a system in short order....

It sounds like you have a good system ...and you do not want temps so low you freeze condensation on your evaporator fins.... I had a Lincoln which started doing that many years ago.. .here in Texas.... once it froze up.. had to wait like 15 minutes for it to thaw before any cold air was available....

__________________
1980 240d ,

chain elongation, cam marks reference:
http://www.peachparts.com/shopforum/diesel-discussion/10414-help-i-need-check-stretch.html

http://www.peachparts.com/shopforum/diesel-discussion/305365-9-degrees-chain-stretch.html

evap fin cleaning:

http://www.peachparts.com/shopforum/diesel-discussion/156207-photo-step-step-post-showing-w123-evaporator-removal-1983-240d-1982-300td.html?highlight=evaporator

A/C thread

http://www.peachparts.com/shopforum/diesel-discussion/297462-c-recommendations-mb-vehicles.html