How can I calculate ambient air pressure from flight data?
It kind of depends on how accurate you want to be because you could start asking about instrument errors and position errors and stuff like that.
But, assuming you don't want to get into that...
The link you included would indeed show a standard atmosphere pressure but the pressure altitude data you have is also referenced to the same standard atmosphere (it actually depends on the setting of the altimeter... if it is set to 29.92 / 1013 mBar then you're in luck).
Ori