Because, rather than using 3/4" ply for both skins of the top's torsion box, we went with 1/4" ply for the underside, we SHOULD HAVE ripped the poplar frame material 1/2" wider to 3" to maintain the desired 4" thickness.
THAT wouldn't be a problem because a 4" wide solid walnut "face frame" would hide the top's narrower thickness. A simple 45 degree miter on the face frame would keep the solid wood look, avoiding any end grain showing.
The left cross section is The Plan, the far right cross section is The As Built and the yellow and blue illustration is the simple mitered corner we would have used IF THE TOP WAS THE SAME THICKNESS AS THE SIDES.
The problem became glaringly apparent after the torsion boxes were put together. I suggested going with 4" wide walnut for the whole face frame, the overhand of the bottom of the top's torsion box would never be noticed. The Designer decided he wanted the top's "face frame" to fit flush top AND bottom. THAT made things a little trickier mitering the corners of the face frame. Here's the problem and a solution.
There are several ways to work out what the "Not 45 Degrees" angle is. Here's the easiest way.
Want the Theoretical / Mathematical way? Then click HERE.