Agree that education is a good thing, but just to add to what has been said:
While the gas goes in on the low side, it is the high side pressure that should be measured to determine when the system is fully charged (with a partial recharge). This value is specific to your AC system, and varies substantially based upon ambient temperature.
That is one reason why it is better to use a real manifold gauge set, as Larry mentioned.
1986 300E 5-Speed 240k mi.