You are correct that most porters assign a certain % to a throat size. That is because it is quite difficult and time consuming to find what works then to manufacture it, especially if bottom cuts are not concentric with the valve seat and top cut. The purpose of the bottom cuts and blending is to transition the air from the port and set it up for the chamber beyond. The chamber is all different shapes and angles. To make the best throat, it has to be customized as well.