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In any AC system, auto or otherwise, you charge from the suction line (low side). The issue with referencing the liquid line (high side) on an auto is that it is always changing. The discharge pressure is important to get a proper charge, but you really need professional charging equipment to get it right, normal gauges arent going to help much unless you are some auto AC savant.
The store bought refrigerant can with the guage simply charge the system to a functional suction pressure range. While this isnt going to give you the aame performance as a professionaly charged system, it will get the job done fine. The only caveat is, if there is 0 pressure in the system DO NOT ADD REFRIGERANT! You will have non-condensables in the lines and can damage the compressor. If you have any positive pressure (gauge moves when you hook it up) you will be ok.
As others have stated, if you have a major leak, you could just be wasting refrigerant, but as long as there is still pressure in the system, adding refrigerant to the safe operating levels on the guage wont hurt anything.
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