I'm not sure what a fixed-orifice system is or how it would matter, but the air coming out of my vents was definitely on the warm side and so I hooked up a gauge to the low side, with the engine idling and the AC on max cool (all of this is per the instructions that came with the gauge) and it read low. I forget how low, but I think was less than 25 pounds and I remember it was definitely in the "charge needed" sector of the gauge. I proceeded to attached a 12 ounce can of 134a with the engine and compressor running and it emptied something more than half (i'm guessing) of the can and now it works fine.

If i remember correctly, he low side should read about 75 psi when the compressor is off but i agree that i think that information is not helpful to determine whether or not the system is low on refrigerant.