Once upon atime, I used to waveski in big surf. A wave ski has 3 fins like a surfboard, but the middle fin(skeg) is adjustable for the following reasons; for manouverability the skeg is placed closer to your backside( centre of gravity) this results in less turning effort required but tracking is reduced, this is good for small waves. Placing the skeg further back, away from the paddlers backside increased tracking and but manouverability is reduced. Better for big waves. I am a novice kayaker but I would guess that the use of a skeg in a kayak would be to help tracking, therefor, I would place the skeg as far rearward as possible.
Hope this helps.