It turns out you can't really sort them all that finely. How do you measure; with rachis flattened? With whatever arc you have put in the rachis? The "length" will vary if you measure with arc applied, but different arcs will yield different lengths from the same overall rachis length. So, a range of rachis lengths in five or six compartments works pretty well. I then sort some for marked rachis curve and some with a shallow curve in different lengths. Most of the feathers in my larger compartment are those that I played with the rachis curve, then decided not to use at the time I made the modification.
Another thing to consider is whether you intend to use the entire length of the feather right down to the quill end or if a feather that is apparently too long will work better than a full length feather when shortened some. It is pretty much impossible to sort for this consideration.