Thank Deja Vu and Coachg for your tips.
I think I have to borrow a 34cm fin from my buddies to see if it works for a 7.0. The day I launched the board was in full power to over power condition. I think a smaller fin would help.
Actually, when I just bought the board, I made my first launch in mild breeze on flat water, the 36cm stock fin plus a 7.0 was a perfect combo at such barely on-power condition. What I can generalize is that, in light air, whatever setting doesn't really make a big difference. When it blows big on choppy condition, higher precision on gear matching is required.
As for the mast track, I am going to try to put that forward to see how the board respond.