I would try it not right at the back but about 2cm from the back (which is around halfway between the middle and the back or 2-3 on Ken's scale). The reason I suggest to go a little further back than Ken has just mentioned is because you are on the '07 Gaastra sails. The '07 sails had the draft quite far back which meant you have a considerable amount of back hand pressure whilst sailing. Whenever I gave someone a go of my sails (especially after riding a NP sail) they would think it terrible that the sails felt so unstable and back hand twitchy, however I thought it such a positive. Having the backhand pressure meant you always had power to load your fin, even in super light winds and you could really jam on the outhaul in highwinds and still have pressure in your sail (the NP's didn't like excessive outhaul).
Anyhow, on the 160 this meant that you were always getting good fin pressure and the board would ride best with the nose quite high. Too far back in the track and I would find with the Gaastra sails it was harder to initiate planing in light winds as you've reduced the wetted surface area of the board with its nose too high. 2cm from the back seemed to be the best compromise for early planing and good speed.
Also, I would probably only use the +8 S in that board. I thought the M was too stiff to get the power and felt a little uncomfortable downwind in higher winds whereas the S allowed you to depower the fin when you are tailwalking downwind
(but you'll know whats best for your style).