The board is foiled from nose to tail so the thickness of the rail is different at 12" from the nose than the midpoint of the board. The rail band chart shows dims for the center of a 2 1/2" thick board. You can measure the thickness of your nose and tail at 12" marks and do a little math to determine the rail marks based on your midpoint rail bands.
After a few boards you can just eye it up though. The most important thing is to keep the rail shape flowing along with the rocker and foil.