If it is 48oz to charge, subtract 8oz for oil. 40oz at 80% = 32oz. Cans are 12oz generally. 2 and 2/3 cans. Your not going to get 100% out of the can. 3 cans is close if 48oz is your fill. You also have to give time for the system to equalize each time you add freon. If not it is easy to overcharge. I don't know if 30 minutes is long enough on the vacuum pump. Depends on cfm of pump and how much moisture is in system.