My little 240 D lets me know when the AC is on... it's like when I turn it on she say's " your gonna regret this on the next hill"...... but she does oK...sorta!

Here is my problem..The original and previous owner spent $1700 on a new AC system put in by German Motors in Las Vegas (last September). I purchased the car in Feb after it took a very light front minor fender hit... BTW, that did not affect the AC.
It just does not feel like it blows real cold on a 92* day. It does get cool but not cold...feels like it's coming out about 55+* at the center vents.
It has R134a in the system... what should it be at the vent using thermometer? Should I top it off with a can of r134A.
Not gonnna go to a shop or dealer cause it 's not that bad... just need a guide of where to start. I'm a AC novice but a good/willing learner and would like to learn from the board. I'd like to preform my own work.

thanks