In both stocks I've made, using points on the action as the basis points from which to measure; and a rough template from the original factory stocks I established rough dimensions. The finish adjustments are done trial and "avoid the irreversible error of taking too much off": Yes, I started with an intentionally larger dimension from which point I repeatedly hold the rifle in position and remove material to establish length of pull, length of trigger pull (grip to trigger), cheek rest, grip size etc. Other dimensions not dependant on shooter fit, are done in a way that will result in practicality and visual appeal (again to the owner). Yesterday I rasped and sanded some adjustments to my last stock to better align my eye with the telescope and improve ergonomics of the comb/grip area.
You might find some basis on which to develop your own stock by typing "rifle stock measurments" with Google search engine. Of course a generic stock can be made if you intend on fitting adjustable hardware on the rear stock. If you want to clone the McMillan stock, as was previously suggested you will have to meet someone who has one or find a picture with a reference point within on which to scale up your own template and to fit your action. Without this, it will literally be from scratch so you will have to do what most of us probably do: Sketch the action measurements on the blank, and considering the dimensions of the factory stock, where does it need to be adjusted to be more comfortable for you and make those changes in the sketch.