Adjusting the Center Fin
There are no exact measurements about where to place a center fin. It really depends on how you surf, what type of waves you're surfing in and more importantly how you want to surf. There are, however, some good guidelines about placing longboard fins.
Start by placing the center fin at the rear base of the fin box at 8 to 8.5 inches from the tail end. If you feel the surfboard slipping on hard turns or on your bottom turns, you can get better hold by placing the center fin back closer towards the tail. Move it back in increments of 1/4" until your slippage is minimized.
If your board feels tight and difficult to turn, place the center fin closer to the nose. Move it forward in increments of 1/4" until you get the desired looseness from your board.