how much freon in a port installed ac system,107

I have a 280 sl 1985 I have installed 4, 12 ounce cans of 134 and I still have a milky sight glass, 160 over 35 and it cools pretty well, but not great. The car was built without ac The afternmarket system was installed in Houston. Most of these add on systems that I have seen only take 2 lbs not 4. I had the evaporator out though and it is huge so I expect it may have a larger capacity. But more than 48 ounces? Help anyone?