Be careful because Kona solids can vary quite a bit (okay, a lot) from dye lot to dye lot of the same color number and thus from the color card. You might want to consider expanding your fabric choices by having the blues fade into creams. It would still have 12 steps, just a slightly bigger step between tints. Moda Bella is also a perfect 'mate' for Kona and I have used it to fill in gaps in gradations on several occasions.
One trick I learned is to save the pictures of the swatches on the Hancocks of Paducah website to my computer and then I can compare colors side by side as if on a design wall. Although the colors may not be true to the actual fabric because of my monitor, they will be pretty true in relationship to each other.