The sphere guidelines are obviously not perfect, disregarding the fact that people are getting diameter and radius confused. I'm sure schema will fix everything soon. Now, for a geometry lesson.
A sphere with a with a radius of 3 cannot be done properly in odd symmetry, it has to be done in even symmetry. That sphere has a diameter of 6. It's point of symmetry is between the 3rd and the 4th blocks, OOO|OOO.
A sphere with a radius of 3.5 can be done in odd symmetry, and has a diameter of 7. It's point of symmetry is in the centre of the 4th block, OOOXOOO.
I would recommened the diameter being the main variable, with the corresponding radius shown nearby.