You may, of course, tangent ratio the design. The requirement is only that the plan view not change. As Jeff Theesfeld says in the cutting notes, the use of a softer material increases the difficulty. The Grand Master stone is supposed to be difficult.
Also, note that the Premaster stone, cut per the instructions, can be in two different forms, depending on which direction your index gear is numbered. (One is the mirror image of the other.) Either form is acceptable.