I don't see why you need distort maps for a scrolling marquee... it sounds like you can do that just moving the objects around.
The top left of the object box is like the top left corner of the selection box in the layout editor - it rotates with the object. The bounding box never rotates, it's an unrotated box big enough to fit in the whole rotated object. Still, if the current calculation works absolutely fine... why are you asking?
Also I'd hope you use a function instead of a macro for that calculation!