First Yes you use the low pressure side to fill the system.

The gauge that goes on the low side if often blue and the high red. The fittings are different so you will know when you get things snapped on right. Be sure if there is a center connector that both valves are firmly closed before you make the connections. Some of the better gauges have a valves that allow you to vacuum both sides of the system by opening both valves. Doing a fill or even a hook up, you do not want that path open. The cheaper gauges only allow the fill port to connect to the low side of the system.

If your system is empty you will want to jumper the low pressure switch and let the compressor run for a minute or so before you start filling. That is to allow the compressor to build up pressure and move the oil around.

When you are ready to start the fill slowly open the yellow or fill hose and the valve on the low side keep your Low side pressure below 40 PSI as you fill the system. If you let the pressure spike it is possible to damage the compressor when the low side jumps up and the high side is at little to no pressure. If you have a set with a sight glass you will be able to see bubbles as you start the fill. A lot of the better older gauges have one.

Now for the variable part of this. The pressure you want to reach is determined by outside Air Temperature and how well your compressor is running. At lets say 90 F air temps you should see 35 to 40 psi on the low and 175 to 200 on the high. The R12 System is supposed to hold 45.8 Oz of coolant. If using F12 or R134 you are supposed to use the 80% rule on fill volume. Do a slow fill and as you reach 35 psi stop filling and check the air temps. If you over fill the system you see a reduction in efficiency. If you still had access to the gauges drive the car for a day and check the pressures again. With any 240 with the older AC system faster speeds on the compressor and air moving across the coils equals better system performance. There are be tables online for Air Temps vs low and high side pressures that you may find useful. Google R12 pressure tables and see if that gets any hits. You can not on an older 240 measure the air temps sitting in the driveway and get a good feel for how well it is really working. You have to burn some gas and check it at speed to really know how well things are working. A system that has a low charge will actually cool better than an over charged one as long a the low pressure switch does not kick in. However the compressor will have to work harder and not last as long. So that is why you want to find that happy medium on the pressure.

Good Luck and hope this helps.

Paul