I wouldn't advise topping an Audi A/C system with freon without evacuation. I'm
on my 3rd compressor; I learned the hard way.
It turns out that MANY things can go wrong and lead you to believe that the
system is low on freon when it actually isn't. For example:
1. The restrictor tube at the inlet of the evaporator can (and does) get
clogged. This gives insufficient cooling, and the compressor really works hard.
Adding more freon can damage the compressor.
2. There is a thermostat in the evaporator that makes the compressor cycle when
the evaporator reaches 32F (to prevent freeze-up). This thermostat fails often
causing the compressor to cycle at "warmer" temperatures. This gives the same
symptom as not enough freon. "It ain't cold enough, add some more." There is a
sight hole in the side of the evaporator housing that you can look in to for
checking ice-up. Remove the rubber cover and look during ~2000RPM. The
thermostat *IS* adjustable. Again, adding more freon will probably damage the
compressor. (I have parts of my blown one here in my office).
The moral of the story is: The only way to know how much freon you have, is to
evacuate the system and add the specified amount. These systems are extremely
sensitive to too much charge. You really can't know how much is in there any
other way. These systems aren't bullet-proof like good old GM systems. There is
no sight glass, and the freon volume is fairly low. The controller is GM/DELCO,
but the rest is Audi. (Yes, I have a Nippondenso compressor too).