The width of the board makes you plane early, not the volume. I'm 84kg and use 7.9 on my 2011 isonic 97. At 70kg i see no reason not to use the 7.8 on your 97, If you want more lightwind performance i would get a 110 or 117w with your 7.8. Just got a 110 myself and it feels small compared to my 127!
Last edited by kapten k; 4th November 2013 at 08:21 PM.