And note also that Scott says the value is no different for used self-adhesives whether on piece or off.
Here's their full statement about self-adhesives, both unused and used:
Unused self-adhesive stamps are valued as appearing undisturbed on their original backing paper. Values for used self-adhesive stamps are for examples either on piece or off piece.
"Understanding the Listings: #9 - Value unused and Value used" (Introduction, Page 14A, 2017 edition)