I assume, as the old ad's used to say, that it's an 'ancient Chinese secret'

They discuss the process a bit in the 'preamble' section of the catalogs without any kind of specifics, so I'm quite sure they keep it to themselves. Given they don't actively sell stamps themselves, I would figure they likely look at dealer 'sell' sheets or any number of other sales records and 'guestimate' the value, otherwise just give it the
italics
treatment due to lack of evidence.. and anything else gets the minimum.
One thing for certain... nothings for certain where Scott catalog values are concerned
