There is an assumption that the same airfoil is constant and proportional the entire length of the wing. This will work in CAD, but I don't always find this to be true. Many A/C manufacturers used a certain airfoil near the root, and blended to a different one near the tip.

The many airfoil software programs have a function to not only alter the blending of the airfoils, but also calculate the overall rib according to chord length then.

Wm.