Don,
There was a time I think I could have worked out the solution for you. My force diagram shows the cable anchored at the root, moving past the gull wing angle (over a bar where it is assumed there is no friction) and on to the tip. The tip will align with the root in line with the direction of the pull, regardless of the amount of deflection represented by the gull wing angle. The pull along that cable is constant at 15 lb. The force trying to straighten that dihedral break is considerably less than the tension on the cable and will be in the neighborhood of the calculations you have.
Now to some side comments. I have had some experience with two different scale swept wing designs where I put the bellcrank in the root of the wing. In both cases, the leadout position, as dictated by the CG had to be well behind the bellcrank position, the the leadouts coming from the wing at nearly mid span position. In one case, I used small pulleys to change the direction of the leadouts from down the wing span to the leadout position. This worked to some degree, but the leadouts still showed excessive wear at the pulleys and one failed at the pulley on a pull test. The other case, I used short smooth steel rods for the leadouts to move over for the direction change from moving down the span to exit at the leading edge at mid span of the wing. This worked for scale where not that many flights are planned. But for stunt, I think the drag over those rods, though not offering much resistance, there was still drag and would not be a good thing for stunt. If the angle change is not much, then you could live with it, but the amount needed to get past that gull wing might be more than you would want. Also, the wear of those leadouts would be a concern. I used the plastic tubing used for flexible pushrod guides to "sheath" the leadouts. I think this reduced the drag somewhat and probably should extend the wear life of the leadouts, but for a stunt model planned to be used for dozens or hundreds of flights, I would still be concerned.
Now, let's talk about Kirk Mullinex and his Coarsair. This is the only picture I have of it and does not do it justice. This is powered by a piped PA 65. It is a large model somewhere approaching 700 sq in. (It might be slightly smaller, but not by much.) Molded shell fuselage, and like all of Kirk's stunters, is light, has a great finish, and flies really well. I think he mounted his bellcrank at the bottom of the gull wing, with the pushrod to a transfer crank in the center of the wing/fuselage. Kirk is a master machinist and he made his own universal joints to operate the flaps across that gull wing. And there is no play between those universal joints. You have to feel the flaps move with no flexing between the joints to believe this was possible. Thought you might be interested.
Keith