Thanks, everyone, for your thoughtful responses. And thanks for delurking, delurked! Your perspective is an interesting one.
I've made a conscious decision to stop worrying about this since without there being an industry standard, or little in the way of published policies, there's little you can do to ensure staying in-bounds short of never returning anything.
I do appreciate knowing that STP's cut-off is 60% (and wish they'd be more straightforward about this) but don't believe that their policy necessarily translates to other companies.
While being cut off can feel personal, it isn't. These are numbers driven decisions. Alas, there's something that feels unfair about them. I'd like to see more transparency in this area.