Hi David,
I learned to fit pistols by understanding the qualities my late brother, John Person, looked for who was an expert competitive shot with everything modern and muzzleloading except a shotgun. Dave Rase is right about the sequence but it is actually difficult to determine the trigger distance until you have a partially shaped stock. The way I do this is first make a good drawing. I base that drawing on dimensions from a good fitting pistol that me or my client has. Often, I don't have that reference so have to start by guessing. Then before I inlet the barrel or lock, I roughly shape the lock panels and upper handle behind the lock leaving the lower handle still squared up. Then I slowly work down the handle to reach a point when my trigger finger easily reaches the line I drew for the trigger based on my plan without the finger touching the stock. This requires experience and some guess work but eventually I have the handle roughed out. Then I determine the lock position and then the barrel. When you shoot a pistol, if your trigger finger contacts the stock very much, as you squeeze the trigger there is a tendency for the gun to move to the right (left if you are left handed). By keeping the finger away from the stock, that movement is eliminated or minimized. The English understood this very well when they fitted dueling pistols.
dave