I am currently in the process of switching jobs doing Res HVAC/R to doing light commercial. Mainly Walkin Coolers, Freezers, Ice machines. All the work will be done in gas stations. The company thats hiring me knows my experience level is residential, and are willing to train me in the light commercial side of things. Now, since central AC is so easy, i feel i've been spoiled after going through my old school books while brushing up on my knowledge. Certain things are making my head spin, and was wondering if i could get some input.

-Head Master Controls are confusing me big time. To my understanding, when charging a Head Master control system, if you charge in the summer time by filling the sight glass, you'll be able to hit your desired head pressure (ambient+30, converted to pressure), and low side (20 degrees colder than box temp) relatively easy. looking for 6-8 super heat on cooler, 4-6 super heat on freezer. However, once the cold weather comes, and the head master control kicks in, my charge won't be right anymore. Now, ive heard various theories on how to charge properly. One way i heard was charge to sight glass in summer, and add 2 lbs per h/p. Another is full sight in summer, plus 20% of capacity.

Now, those charging methods are done by adding to exising refrigerant. For example, leak in evaporator. Pump down into receiver, change evap, pull vaccum, and then add to sight glass till clear and either add 2 lbs per h/p or 20% of capacity. Is any of this true? Is it possible to add to existing refrigerant with head master, or do you have to evacuate system and then weight it all in?

Also, while charging Residential Central Air, charging is done by sub cooling if unit has TXV, super heat if unit has fixed orrifice. Checking superheat back at compressor and sub cooling at outlet of compressor, while checking humidity with psychrometer and outside temp. Is this still true with package AC and light commercial split AC systems?

Thank you!!