Here's how I see them from lightest to darkest.
Lights - 7, 8, 21, 24 and 4
Mediums - 26, 16, 17, 18, 22, 23, 13, 14 and 25
Darks - 12, 15, 2, 3, 5, 19, 20, 6, 9, 1, 11 and 10
Obviously, the delineation between what is considered a light vs. medium and medium vs. dark is subjective. The lighter mediums could perhaps be considered lights and the lighter darks considered meduims to even out the 3 categories.