To position your actor, you can use origin, scale, and orientation properties of the actor.
As far as I know, UserTransform in the actor is a very low-level API. If you use us then you are expected to take care of all consequences. UserMatrix might work a bit better.
To VTK developers: It would be nice if VTK documentation would clarify what is expected when someone modifies an actor’s UserTransform or UserMatrix.