Some Sub Zeros require a higher low side pressure.
At sea level, most small appliance refrigerators charge at 4 or 5 psig. If you are at mile high level (Denver) then you charge to around 0 psig. Your gauge measures relative to outside air pressure. At higher elevations, the pressure is less, therefore your gauge reads less psi.
The best method of charging is superheat. It's the only way to ensure you are not getting any liquid refrigerant back to the compressor.
Does anyone know the acceptable superheat charge for small appliances??