If you have to have the XY plane (the pivot) coincident where you show it it seems to me you'd have to know the angle of that slope relative to the top, subtract it from 30 and use the result for the angle from the flat top.
Also not sure why you care where the "pivot" is. This is one of those cases where this might be easier in sync. At least in terms of getting the angle and getting the plane coincident with that line without needing calculations.
If you want to stay in ordered. I find creating sketches on the typical planes the easiest first step. then create a plane either with 3 point or normal to curve. The sketch allows you to have control over the plane angle.
I have given up using planes (which is probably bad in general) for angled work. I usually use a sketch which I extrude to form a surface. This automatically creates something I can draw on and attach to for assembly purposes, and I don't have to go through the pain of making planes.
Create the angled plane to the top surface then create a plane parallel to it, using the front corner of the sloping surface to define the offset distance.
Use the Angled Plane command like you were. Select the FLAT plane as the parent. At the step for "Edge to Pivot About" (or whatever it says), pick the vertical end face. It will then create a virtual edge at the intersection of the horizontal and vertical planes and pivot 30 degrees about that. Shoudl be what you are after.
Sure. Then you do a parallel plane through the edge in question if that is important. This is of course the issue in ordered -- you have to have a "recomputable" set of steps and therefore constructs can sometimes be a bit more involved.
When I use angled planes, I typically need to go back and move the pivot point, angle, and position as part of the design cycle. This is why I like a sketch to be in control allowing complete freedom of the planes position without losing at references like includes and parallel relation from the plane. If I picked anything I can't move as the pivot point....I would have to start over. Anything downstream becomes lost.