I'm no expert on the adjustable 4-link setup
but i am under the impression that the optimum angle will change alot with individual car setup
so there is no precise measurement that is the "best"
stuff like spring rate, sway bars, condition of bushes, tyre choice (profile etc) will all have an effect on where the "optimum" position for your diff is
i think the best way to go about it, is experimentation with your specific setup