I'm not sure if a Part 27 rotorcraft will use exactly the same procedure and guidance as a FAR 25 fixed wing, but I'm referring to AC 25.562-1 for my answer.

The deformation of the beam is made relative to the original position, per Figure 9-1. That makes it pretty clear to me that the original orientation of the beam, either in parallel with the aircraft horizontal plane or at an angle, is the starting point. This is a little different from the text of paragraph 9, therefore I can see how you might be seeking clarification. However, by considering that the vast majority, if not precisely all, fixed-wing transport category aircraft mount their floors horizontally, then it's no surprise that they would mix up "horizontal" with "longitudinal axis" as equivalent. Some exception in a different category of aircraft is not going to cross the minds of the people who wrote the guidance.

Sparweb,

Thank you for your answer.
I will proceed as you described since it is also stated in AS8049:
"The roll beam shall be capable of ±10 degrees roll about the centerline of floor tracks or fittings"

Oded