@Mbarber866
Perhaps the Chamfer could be created before attaching the Cube shape being modified?
However looking at the outer top edge of the ‘Cube’ it appears to have a curve and it is obviously longer than the inner edge.
Another way to remove the Chamfer is as follows.
Add > Construction Plane > choose option Through Three Points:
Above a Rectangle has been created on the Construction Plane to encompass the appropriate Edge.
The edge nearest to the Main Body will have to be precisely positioned.
Next select the Rectangle and pull the Arrow upward in one movement:
Leaving:
To use the Construction Plane in the manner you suggested it will require another CP at 90º and 90º to the Edge being modified.