Not always easy to tell just from the operating pressures as the system will try to self protect and adjust bypass valves etc to maintain the reference temperatures. The measurement of the pipe lengths needs to be a good estimate and not precise to the last mm. This should give you the best guide for the gas charge. I have never used the self charge as it can take quite a while to complete.
Are the four cassettes with poor td's at the end of the line? If so the system could still be sog.