3D Geometry: Drawing line with a specific gradient on a radial surface?

Hi all, I'm in need of a bit of workflow input...

I've got a large wall with an outer radius of 78 metres( 13metres high, 500mm thick)  that I need to place a 2:1 gradient cut along (the cut will start at 775mm from the base of the end of the curved solid) but I am unsure how to achieve this in MS. I know how to work out what the cut height should be along the wall at specific intervals and I could then draw a b-spline curve along the wall, what I don't know is what I would do after to make cutting solid that would remove the geometry above the cut line on the solid, or even if there is any point in doing it that way if there is a tool that will make the job far easier. 

Any thoughts would be appreciated.

Thanks

PS I posted in here as I felt the topic is more appropriate than in the visualization section as its not related to rendering etc... but feel free to move it if necessary.