That method of charging will work (somewhat) and was common when systems were charged with r12 freon years ago. You add until the low side pressure is what YOU thought is ok. And you can still do that with r134 with varying results.

Refrigerant is put in by weight. Approx 1 kg. (specs are usually written under the hood) for a full charge. With r134 too much or too little does the same thing - poor or no cooling. R12 was not as picky and would still work ok under or over charged. That silver can in the yellow (low side) line is used to inject dye or oil into the low side.

 

A lot to learn here! I believe the mechanic did it by looking at the pressure. It was ice cold once he finished but a week or so later can’t say that’s still the case although it is cold.