For your AC itself, how often is your compressor cycling on and off? I use that observation as a crude barometer for how charged it is. If you are cycling every few seconds, that indicates that you might be down a little bit. Even if it stays on pretty long at idle (before disengaging), hold the RPM's up and watch the clutch. See how long it takes to disengage the clutch. Basically, when the compressor engages, the low side is pulled down till it hits the threshold of the LPCS, and the switch kicks the compressor off. The high and low side start back towards equilibrium until the clutch engages again (and the low side is then pulled down. The cycle is recursive).
Here's a
pressure-temperature chart. Basically, the pressure you see depends upon ambient temps (if you drove out here and tested you car in 110*F, it would read differently).