29.9 is not perfect....
the barometric pressure varies....and 2 niner niner 2 is the reading on a ' standard ' day... at sea level at a specified temperature...
So perfect on any given day has lots of variables to factor in... but most techs have a reading on their machine which they quit once reached...
Vstech is referring to the fact that our AC's have a controlled leak at the compressor.. for lubricating the compressor front bearing.. making this NOT a Hermetically sealed system the way a refrigerator or your home AC system is... the compressor sitting in oil and all the lines soldered or the like...
I do not know if Manual Life has these answers to give us.... or even if he can get them from his friend... it is a good chance to go over some of the basics though...important basics...

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1980 240d ,

chain elongation, cam marks reference:
http://www.peachparts.com/shopforum/showthread.php?threadid=10414

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