Not sue if it is what you need but maybe this article could be helpful:
https://dof.robotiq.com/discussion/2365/align-part-surface-with-a-plane-using-robotiq-force-copilot#latest
It presents how to align an object on a surface using the FT300.
If the robot is closed on the object and if the object is well aligned with the base surface then you know its normal.
I have a problem where a threaded cap needs to be operated using FT300 sensor mounted on a UR robot automatically. Since the cap pose can't be precisely identified so the FT sensor needs to identify the normal of cap using compliant mode. I want to know how to proceed in this direction.