Agree. I looked carefully at your installation diagram where the 360 degree servo rotary movement is transferred to a plus/minus shaft. I didn't expect to change rotary direction of the SlimJim but the first movement of the drive pin away from centreline would be in the opposite sense. If there is a flat on the SJ shaft that denies the change there, the opposite crank can be introduced at the rudder end of the drive rod.
I don't think I am having a Brain fart !