What if outdoor temperature is very low during charging?

Q: How do I check the refrigerant charge with superheat or subcooling?

On a fixed-orifice system, measure suction pressure and suction-line temperature: superheat equals the suction-line temp minus the saturated-vapor (dew) temperature at that pressure, and the tool compares it against a target derived from indoor wet-bulb and outdoor dry-bulb. On a TXV system, measure liquid-line pressure and temperature: subcooling equals the saturated-liquid (bubble) temperature minus the liquid-line temp, compared against the manufacturer target. The unit charging chart is always authoritative; this is a field cross-check.