Your mounting position is determined by two factors: the direction that the populated side of the board faces, and the direction of the power cable. Both of these should be considered from the point-of-view of the camera. Some examples are below.
Please note that in these example pictures, the HoverflyGIMBAL has only been tacked on temporarily with Velcro.
On your build, you should attempt to mount the HoverflyGIMBAL firmly and square with the level position of the camera tray.
Note that if your Roll and Pitch servos are continuous rotation servos, your HoverflyGIMBAL MUST be mounted to the camera tray such that it moves with the camera. Without the benefit of servo potentiometers, the HoverflyGIMBAL must rely on its own position to determine the position of the camera.