Am I correct in thinking that you have already built the table top and are now finishing it off with the detail work? Or have you not yet started making the top? If you have already made the top, then the next question is, have you flattened the top? If the top is flattened, then that can be used as a reference. Do you have a normal router plane (70 or 71)? If you do, then set that to mark out the bottom edge of your groove and mark all the way around. Mark the upper edge of the groove with a normal gauge, and make the groove a bit wider than the cutter in the plough plane /chisel. Set the fence of the plough plane so that you are cutting to the upper gauge line. Plough two of the corners along the grain to relieve the breakout that might occur. Swap the fence to the other side of the plane and then plough the end grain towards the already ploughed corner. Once the groove is ploughed, come back with the router plane and use that to pare away the bottom side of the groove. People will notice this edge the most and the router will give you a really nice consistent cut, they will pay less attention to the top edge of the groove, and you can always break that edge with a plane to give the illusion of perfection. Reset the plough plane fence again and do the other two sides. Then set the router or the other gauge deeper to mark the top edge of the bevel. Flip the top over and plane the bevel. Again, pay attention to the gauge line, as any inconsistencies will be quite visible. Don’t worry about the underside of the table, no one will be looking there.
Colin, Czech Rep.