The foil (thickess) of a board is all dependent on the midpoint thickness and volume of foam you need to float.
Most blanks have the foil already shaped in and not much additional tweaking is needed, unless you choose a blank that is not right for the shape.
If you're making a 3" thick board at the middle, the thickness at 12" from the nose and tail will be somewhere in the 1 1/2" to 2" thick range.
There is not right or wrong numbers. It all works.
Here is a reference drawing for the how the rail band dimensions change along the foil of the board that may help you see the proportions of thickness.
You can measure the thickness of your current surfboards as a guide too.