We used a 3/8" spacer fence on the router table with a 3/8" spiral bit. The space between the rail and the router bit must be 3/8". Set the height of the bit the same as the thickness of the stock you are using (ie. 1/2" stock, set the bit height at 1/2"). Use sample wood and run a test piece to check the fit.
You can use any angle you want, just make sure that it is exactly the same on both the front and bottom of your project. On our clock project the angle was approximately 30 degrees.
You will also need a push block to keep the stock in position when cutting. This also prevents chip out. The angle cut must be the same on both ends of the push block and it must be exactly the same angle as the ends of the project pieces.
To make sure the box joints match up you need to make a pencil mark on one edge of both the front and bottom piece. Make sure this marked edge is against the rail of the spacer fence when cutting.