Bluebook is known to be conservative. Percentages are subjective. Gunbroker advanced search of completed auctions will provide upper bound of value. Providing a value estimate via two photos is a fool’s errand.
If you're trying to sell a PP series Walther at your LGS and they look it up in the blue book, you'll most likely receive a greatly devalued appraisal of its value. The blue book is a hand wave that does address the many factors affecting value, nor does it cover the many variants of any given pistol.
This can also work in your favor when you find a Walther gem at your LGS for several hundred dollars.