Let me preface this reply with the fact that water doesn't know numbers, it's the flow of shape that matters. Numbers are only a guideline to shaping what looks good to our eyes and feels good to our hands.
That being said the railband dimensions change at 12" from the nose and 12" from the tail due to the tapering foil of the blank's profile and the narrower nose and tail widths than the midpoint.
Most importantly you want to copy the bottom rocker onto the side rail and make that taper slightly in the nose and tail areas. The deck rail bands will affect overall volume of the board and shape of the top rail.
You can do math to calculate the rail mark dimensions at the nose and tail based the change in thickness compared to the midpoint. For example, a 2 1/2" thick midpoint rail mark of 1 5/8" [1.63 / 2.5 = ~60% for a 60/40 rail] will reduce down to 1" if the foil thickness is 1 5/8" @ 12" from the nose (~60%)
You can put your rail marks on the board and trace them out (making a tool for that is fine but I use my hand and drag my pinkie along the edge or bottom to trace the outline or rocker and adjust the curve while walking to hit the rail mark dots - or come close.
Then take a look and if you don't like it, make adjustments. It'll all shape off when tools hit the foam.
The apex is between the rail mark and top of tuck and will appear when screening the rail bands to round them off.