Since IK Multimedia doesn’t provide any specific guidelines for this (at least I couldn’t find anything in the manual), you have to assume that the measurement is always taken straight ahead.
While other measurement systems generally also allow for measurements at a 90° angle upward, this is not explicitly mentioned anywhere, so one must assume that the 0° measurement with the microphone facing forward is the intended configuration.
You’re right: Sending a request to support (and posting their response in this thread) would still be really helpful!
I actually have the impression myself that the setup with the forward-facing microphone leads to some strange results, especially in a surround sound setup.
Perhaps there’s a technique recommended by the manufacturer that they simply forgot to mention in their manual…