Alphabetical, without the need to keep moving them around: I limit myself to 1 in/1 out, OR, even better 1 in/2 out and limited myself to 18 lf total (actually 17lf lps + 12 dividers/6/row). roughly 8 to an inch max, best to leave some space
2 rows 9 feet long at easy height, one just above, 1 just below standing eyeball height, no bending, no reaching,
I have alphabet dividers,
https://www.amazon.com/dp/B07CYN8SRN?psc=1
I installed 8lf of strip light overhead, close so you don’t make a shadow.

note: mine are hardwood library shelves I inherited with recessed metal tracks for thin adjustable dividers. I put them on top of the case’s shelves, and they are only 11" deep, so you can get them in/out more easily
One way to prevent sag is to limit the width to 30" wide case, so the shelf is only +/- 28" long


