Orthogonal technically means "at right angles" but I've heard the term used loosely to mean what I was driving at: providing maximum coverage of the problem set with the least amount of overlap. Conceptually, logical right angles might allow one to compose a two-element query where each element can be adjusted independently with linear variation in the results. Conceptually linear? That's why it's more a contemplation, a notion, rather than a rigorous proof. These thoughts lead to what I can call my best educated guess.
After much contemplation, I have my predicates.