I've ridden the infinity 76 and the infinity 99.
For the 99 you will in all likelihood have the most success with the fuse in the "C" position.
For the 76, if you ever ride that, you'll likely want it in the "B" position.
The pamphlet that came with the slingshot kit didn't really have it right. They say "A" for beginner. As a beginner, I could not get anything going until I got the foil in the B or C position.
The 99 has a lot of area on the aft side of the wing compared to their others, so you need to push it forward to get the lift more forward and centered between your feet.
The best video I found for balanced, stable flights was actually this short one on going fast. He describes the hip movements. Before that, I was doing something a bit more goofy: