Isn't the depth of the drop determined by the playcall and timing with the routes? I've always understood it to be that there are 3, 5, and 7 step drops depending on the how long you want the routes to develop. Maybe I'm wrong.
There are definitely calls for 3, 5 and 7 step drops but that is when the QB is under center. Doesn't work the same way out of the pistol/shotgun.
Are you sure? I'm about 85% sure that a big part of Taylor's work with Calhoun, as reported, was his drops in shotgun/pistol both on straight passes and on play actions, and getting the footwork and drop depths right.
A big part of it was that his first two years his first step back was with his left foot instead of his right, which turned his upper body the wrong direction than conducive for passing the ball, and also made him have to take an extra step to progress through his dropback.