I just received a fully functional 3 Ton AC unit from a friend that installed a totally new system. I have the compressor, air handler and evaporator. My question is on recharging, how do I do it? I have a set of R12 gauges and a Vacuum unit, also got through a friend. My current plan is to pull a vacuum on the system to about 28" and install Envirosafe 22a to the low side. But, how much do i put in? Since I have no documentation on the unit what pressure on the low side is enough? How much refrigerant should I buy?