Yeah, I can see how there would be issues using the conventional method. So in your case, you created a wall and shaped it, added a cap and all was good?
I guess in that sense, I could use a cad line on to connect nosings to get the angle (used as a guide line), then move it up to where it needs to be in order to get the proper wall angle that the top should be and adjust the wall shape to that cad line?
What's your plan for the missing cap in the corner at the top of the stairs?