If pressure/temperature measurement of state of charge is lame then I and many others in the HVAC industry are screwed. You can charge via pressure / temp just fine in most systems. (Some such as small cap tube systems often seen on cheap cube fridges are better total charged and if the system wad dead empty you should leak test and pull a vacuum first) I do HVAC work every day this time of the year and test and charge via pressure and temperature all the time. Humidity has little influence unless you have a evaporative cooling tower on the roof. When you have 45 pounds of R22 in a single loop and 6 loops to a singe roof unit and 5 roof units on one roof you can't pull and charge via weight. 
Read the directions and follow to the letter.
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