No, quantity is best measured from a known starting point, i.e. 0 and
then adding it by weight.  Saab (and everyone else) list the charge by
weight, not by the low pressure gauge reading.

David.

fairly good or correct?, there's a difference.  One is correct, the
other can be either too little gas and consequently too little
lubricant, the other is too much gas and high pressures leading to a
stressed compressor and potential premature failure.

I know you can get the kits in Walmart etc and i'm sure many people are
perfectly happy with them.  All i'm saying is that there's the
guesstimate/estimate whatever you want to call it and then there's the
proper way as specified by the manufacturers which is why the people
that do it properly, use scales and not just a pair of gauges.

Incidentally, not all cars have a high pressure gauge take off, Volvo
850 for example thus making the top-up concept even harder.

FWIW, I recently had my 9000 re-gassed, not that I suspected anything
wrong with it, just that I didn't know the history of the A/C and wanted
to have a known datum.  My usual chap came round and connected the
gauges and was pretty horrified by the high pressure shooting round to
400psi and what he thought was an audibly straining compressor.  

As an experiment, he emptied the system and measured the recovered
amount which was 1.2kg when the proper charge should be 950g.

Hopefully, i've spared my compressor a premature death.  If people want
to go ahead and guesstimate, that's just fine by me, it's not my money
that's going to go on a new compressor. :)

David.