Hi Ted
I've posted this somewhere on here before but here we go agian .
If you can see the top of your receiver dried check to see if it has a sight glass if so add refrigerant to the running system at a good idle ( 750 to 850 RPM)
Add refrigerant untill the bubbles stop showing in the sight glass and then add another 1/2 pound of refrigerant.
You may need a light to see the bubbles and remember a clear sight glass means the system is either completely empty or very low or the system is full.
Vent temperatures are reliant on air flow through both the condenser and evaporator and your system I believe uses a trinary switch to control a relay for the condenser fan your system performances can be unhanded by sitting the relay to run the condenser fan full time when A/C is on this allows the system to move more heat away and there for lows vent temps as well.
This has been a long standing rule of thumb and hasn't let me done yet.
Good luck
Eric