Can't say how much time passed sitting, staring at the chair dry fit in this position, before the epiphany struck me. I was contemplating the seemingly impossible strategy for putting in two rails between the front & back stretchers. Never before had something so difficult been attempted in the history of bunkhaus. All kinds of angled joinery...on both ends... That's when it hit me. I needed a dedicated angle-finding gimp. A gimp with a predisposition for detailed work, that I could bring out when this type of situation arose. If only my shop had a basement...
In the meantime, all this thinking about gimps prompted my second epiphany; the bevel I had made on the top of the front stretcher would make the perfect starting point, a solid foundation upon which to build. Yes, this is how the angle-finding gimp would do it. To find the compound angle for the curved rear, first the horizontal angle, using the highly technical method of drawing a line on my bench:
Then the vertical angle, using the equally technical method of clamping the stock to the foundation, the bevel on the front rail. The angle on the front of the rail would be the foundation bevel angle, of course.
Then it was a matter of precisely cutting the test piece to length. I started by cutting the rear compound angle on a test piece, and making the front angled cut on a trial & error basis, using crazy glue to stick small pieces of wood back on to the test piece whenever I got too greedy.
Cut the rear on the live stock and transfer the length from the test piece, being careful to use consistent corners of measurement.