The pressure is mostly dependent on the temperature outside. The gauge (comes w can) will have temperatures and corresponding pressures. If I remember the instructions: Shake can, Set dial to the appropriate temperature, Connect to the correct fitting (High and low pressure but it only fits on the appropriate fitting), turn upside down and with a/c compressor running on high pull trigger (Where gloves in case of leak). Stop when pressure is at desired. The line should be sweating. You can buy cans that state they swell the seals and help stop leakage.