Here is an excerpt from the APPC Pro manual on pressure. The highlight below is mine. MGPV2 Box will send local pressure to APPC via the astromi.ch ASCOM driver.
Pressure: Here you can enter the approximate atmospheric pressure. This is used for refraction
calculations. The atmospheric pressure varies based on two principal factors: the weather, and your
altitude above sea level. Note that this differs from typical weather forecast "barometric pressure" which
is the atmospheric pressure normalized to sea level. Do not use barometric pressure values. Light is
being refracted by the actual air mass above your telescope, not the equivalent normalized sea-level air
mass. As an example: mean sea-level atmospheric pressure = average barometric pressure = 1013
mB. The Astro-Physics observatory is at an altitude of about 228 meters above sea level - none too
high. The sea-level value of 1013 mB is equivalent to about 987 mB of actual atmospheric pressure here
at AP. At a remote observatory in Chile at an altitude of 2295 meters, that same sea-level value is
equivalent to about 770 mB.
For any given altitude, the atmospheric pressure will have a certain range of variability that can be
expected. Within the range at your observing spot's altitude, you will find that higher pressure
corresponds with nights where it is clear enough to actually use the telescope, and lower pressure
corresponds with cloudy or even stormy weather. When manually entering a value here, enter one that
corresponds with the high pressure value for your altitude that you would be likely to find on a good clear
night. In a future release, APCC will add support for weather station software so that this value can be
kept updated in real time.