Comment by tempestn

Ah yes. I thought through it picturing the diagram in my head rather than looking at it. In my mental picture the board was sitting on top of the edge of the top floor, rather than coming up to flush with it. Which obviously doesn't make sense in retrospect!

What's coming to mind next is a horizontal cut as first described (placing at the angle you want). Flip, then vertical cut, and finally trim the end with the horizontal cut by making a parallel cut offset by the amount the board extends above the upper floor. I'm sure there's a better way though. Non-trivial indeed. :)