Can't figure out how to cut compound angle
An alternative method is to insert a Plane and use an Extruded Cut
Create a Plane. (Features, Reference Geometry, Plane) select 3 points on the sketch,
Start a new sketch and draw a large rectangel on the plane.
Use the rectanle to perform an Extrude Cut (use the Reverse Direction option)
Repeat these steps to cut the other plane.