Adrian it is sometimes useful to also read the documentation for the devices and if you did so you would find out what exactly MB does, I highlighted very important part:
Pressure Correction
In this field you can add an offset to be applied to station pressure in order to calibrate pressure measurements. Offset can be specified in mbar/hPa or inHg, according to the unit you select with the drop-down box on the right. Offset will be applied on top of altitude-based pressure corrections.
Pressure Correction
In this field you can add an offset to be applied to station pressure in order to calibrate pressure measurements. Offset can be specified in mbar/hPa or inHg, according to the unit you select with the drop-down box on the right. Offset will be applied on top of altitude-based pressure corrections.