[Big Kev-O]

"they will blow 30* air at the vents"

For the record your cars AC should blow air around 40-42 degrees. IF your vents are putting out 30* air your refrigerant charge is not correct, or you have a broken thermometer. When an AC evaporator is under 32* it begins to freeze the moisture in the air and turn your evaporator into a huge ice block which prevents air from flowing across the coil. You also risk slugging the compressor with liquid.

You should not try and attempt to charge your system to achieve 30* air out of the vents.