Chris,
As you can see,there are many ways to do this.Here's another..use chamfer,but set your option to "angle and setback"(the 1st button on the ribbon bar) you'll need to extend your chamfer on the 4 sides to the 2nd plane,because this adds material to your part,choose the edge to chamfer as your short red line(side opposite I suppose).You'll need to know the distance and the angle.
Hope this helps