Thanks everyone. Finally got an 80 degree day. Added about 9 more ounces of 134a for a total of about 28 ounces. Low side is 22 and high side is 180. Vent temp is about 43. The static pressure at ambient is about right. I'm pretty sure that the evaporator temperature control switch is cycling the compressor, so it's probably not going to get any colder. The low side still seems low, but I definitely don't want to over charge it. Any thoughts? Or should I admit defeat and have a professional fine tune it?