I'm going to be brutally blunt, here. I spent about 15 years servicing low, med, and high temp AC and refer units.

You should SERIOUSLY consider hiring someone who knows what they are doing. Refrigerant is nothing to play with. It can seriously injure you, and mistakes in charging--like slugging the compressor with liquid--can turn it into instant junk. Undercharging can be as bad as overcharging, and especially if there is some problem in the system, novices tend to "add more" to see if it will cool.

Once again, a good way to scatter a compressor

240, by the way, is pretty high for a target head pressure with R12. My old Ranger still uses R12, and as i remember, it runs around 180.