I used to do all my own A/C back in the days of R-12, but have found there
is no reliable way of making the charge anywhere near correct except by
evacuating and adding the correct amount by weight (just as the experts
say... imagine that!) Guage readings (hi/lo) combined with knowing the
temperature and humidity can give decent charge levels with R-12, but not
with R-134a. Dunno why.
I recommend taking it to a professional for service. You will be ahead in
the long run, especially since overcharge can damage the compressor. That's
true for R-12 also, but it's a whole lot easier to avoid overcharge with
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