Revit floor - aligned to angled plane

Does anybody have an idea of how to adjust a “real” Revit floor to an angled planar surface?
I am trying to find the correct definition of the slope arrow adjusted to the wanted geometry, but somehow I am not able to make it fit. Maybe it is my understanding of how the slope arrow works - i get “close”, but never precise. Or maybe there is another clever way?

Green - vertical floor, Red - wanted planar surface.

Best regards Rasmus

You can add points to the floor surface, then select the points and adjust the elevation. We don’t have an automated way of doing this in RIR yet