Be aware that there are hard edged grads and soft edged grads.
The hard edge grads are usually better for telephoto lenses and the soft edge are better for W/A use. When in doubt, go with the soft edged grad
You could buy a set of soft edge N3, N6, N9; a hard edge set of N3, N6 & N9; a soft edge set of blue N3, blue N6 & blue N9 plus hard edge blue N3, blue N6, blue N9 plus, plus, plus, plus…