Dear Jack,
When you purchase a shirt, you measure around your neck where the collar turns over. But when you make a shirt, you measure around the base of your neck, add 1-1/4 inches, plus seam allowances. This gives you the proper size for the stand, including the allowance for buttonhole and underlap. The collar is then drafted to fit to the center fronts (that's why you have to measure at the BASE of your neck when drafting). In times when really wide neckties are in style, the collar is cut back even further.
If you're using a commercial pattern, you must adjust the neckline on the body. Then, it's safe to enlarge the stand and collar at the center back.
A further note. If you have an athletic type neck, you may want to slash the collar in about three places, and open the free, outside edge of the collar (without changing the neck edge), so that the collar lays flat when buttoned. I'm sure you've seen muscular men with their collars curling upwards--this is solved by slashing the outer edges until it lays flat.
Teri