You're not limiting the top or the bottom..... you're limiting the total degrees of mechanical advance so your creating a window you set the bottom by turning the distributor, the top is the total amount of mechanical advance.
So if you want 2* initial and 20 total you would use the 18* slot, don't know of any engine that can run on 2*, most like 16-34 to we have that option in the plate being 0-10-12-14-16-18
