I used the same board in 2011 on the PWA. I was 80kg at the time. I think us lighter riders need to work to get more weight on the back of the board over the fin which a heavier rider can do more easily. So I have my back straps in the back holes and my front straps in the middle holes.
Depending on your height, you can change the distance between your straps by just moving the front ones forward/back: having them in the back holes (same as back straps) gives you a nice comfortable stance. Having them in the front holes (back straps still in the back) gives you a very wide stance which might feel weird at first but you can have much more control over the board in stronger winds as you have more leverage over the pitch of the nose of the board.
Mast track I had generally in the middle or 1cm behind middle. I saw other riders like Alberto Menegatti putting their mast track all the way at the back, but I found my board would fly away when I did this.
46 and 52 fins will be perfect. I used 46 with my 8.6m nearly all the time for downwind slalom. For cruising and with the 9.3m, the 52 will be nice.
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