If all you are wanting is an angled edge for you slab as you show in your custom profile, you can do that directly by adjusting the edge of the slab.
Place your cursor in plan or 3D window on the edge you want to modify and click.
In the pop-up palette, select the edge modifier, the last button on the top right.
Adjust that edge or all the edges as desired.
As to your original question, hard to tell from your notes and screenshots. They can be buggy. Try adjusting the angle to less or more to see if that is the problem.
Erika
Architect, Consultant
MacBook Pro Retina, 15-inch Yosemite 2.8 GHz Intel Core i7 16 GB 1600 MHz DDR3
Mac OSX 10.11.1
AC5-18
Onuma System
"Implementing Successful Building Information Modeling"