Not having a minimum scares me a bit. Zero is better IMO than a short one.Close. <2' drop, whatever you want. More than 5'10" drop, 42" railing. Between those two, 36" railing.
The stupid bench railing combo turns almost every situation into a >2' drop that requires a railing 36" above the bench, not the 18" that is typically there.
Let's assume a drop of under 2 feet and someone starts going over the edge. With NO railing they can hopefully get their feet into a position where they can come down in some type of stumble. Put in a a railing below knee height and they can't get their feet out to where it would do any good. Basically a trip wire and they come down hard on their torso or arms.