I'd think you'd want to know a bit more about the gun's purpose. target--offhand or prone specific, hunting--small bore big bore, and gun weight, what kind of game, weather conditions---how much clothing (padding) All of these could have an impact on the stock design requirement for a custom gun. Also the customers face structure effects how his head and the stock must be positioned to get the right sight line.
Having a selection to try is a big help. As does actually shooting; however shouldering and or shooting a bunch of different guns might also throw a bunch of additional unwanted variables into the customers mind. a "best gun"-type stocker would have an adjustable try gun since that would rule out all the variables except the configuration of the stock itself.
In addition we also are faced with the fact that many of the guns that interest us (and probably, our customers) represent historic school/styles that have some rather tightly specific stock configurations. Messing with them by altering drop at heel, comb height, even length of pull (by very much) could create a rather odd looking rifle.
I can imagine that creating a big bore heavy recoiling rifle in one of the slender, deeply-dropped, sharply crescented-buttplate styles would be an uncomfortably shooting gun. the same stock design in in a smaller bore target or small game rifle might work perfectly
