I don't have the high side fittings on two of my cars converted to 134 fittings because of what you say, high side has a mind of its own. I go by just touch of hand on the suction line at the compressor and monitoring the site glass. On my Rampage if the site glass goes completely clear I have over charged and can feel the temp change on the suction line. Scientific approach to A/C servicing? No, but it works for me.