Hi unregistered, I am around the same weight and height as you and I use the middle set on both the forward/aft and inboard/outboard setting and the mast base position bang in the centre and this worked for me. Maybe if you are doing more blasting in flatter conditions then the outboard strap position should be considered. As for fin sizes, I have only used this board with a 6.7 to date so cannot really say for sure but am guessing that the purchase of a slightly larger fin for the 7.5 would help.
2011 Neil Pryde Atlas 4.2 & 4.7, 2012 Firefly 5.3, 5.7 & 6.1, 2015 Ryde 7.5 & 6.5, 2014 Severne Overdrives 7.0, 7.8, 8.6 & 9.5
2015 Kode 86L Freewave, 2016 Flare 103L, 2015 isonic 107 & 120, 2014 AtomIQ 110 & 2014 Starboard Ultrasonic 147