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To understand Subcooling, imagine a system empty or nearly empty of refrigerant. As you put refrigerant into the system, once the liquid line temperature becomes higher than your hand, you will feel it slowly rising as the pressure on the gauge continues to rise. It will continue rising and may even feel very hot, but at some point near a correct charge of refrigerant, it will simply STOP RISING even though you continue to add more refrigerant and the pressure continues to go up. In fact, as you do so, it will actually start decreasing. THIS COOLING OFF BELOW THE GAUGE TEMPERATURE CONVERSION IS SUBCOOLING. At this point you can know that the liquid line you are measuring will have a full flow of LIQUID REFRIGERANT in it rather than gas and bubbles. This is a necessary state so the flow control device in the evaporator (TXV or capillary tubes) can have a full flow at its design point.

Thermostatic Expansion valves will work perfectly at a full liquid supply to it at very low pressures, but capillary tubes are dependent not only on a full liquid flow but also the pressure of that flowing refrigerant. So the subcooling may go quite a bit higher on capillary or flow-restriction devices than it will on the TXV.

The amount of subcooling for TXV systems may range between 10 and 15°F. and this is perfect for the successful flow of liquid at the TXV and it should function perfectly unless you have extra long refrigerant lines leading to the evaporator or it is extremely high in elevation above the condenser. Every fully charged system will have subcooling.

Capillary tube systems may have 15 to 30°F subcooling when they are working right. Remember they are also dependent on the pressure of the refrigerant to "force" the liquid through a restrictor. Determine the correct refrigerant charge on them by relying more on the Superheat once you are in-range with the Subcooling. If you are this close, you are within the "ball park". That's where I want you to be. Just close, not perfect. Your experienced technician will make it perfect when he looks at it later.