How do I set up my snowboard bindings (what degrees) to ease freeSTYLE strain on my post-ACL surgery knee?

The closer to the middle the bindings are, the less stress on the lift, but you are into freestyle, which requires a wider stance. If I were you. I would just go for some bindings with canted foot beds, (Rome 390 Boss, Ride Contraband) to name some. They will align your knees to relieve the pressure off of them, for a more comfortable ride. The angle positioning shouldn't make too much of a difference