Mercs. FB And Not FB

I am sure this has come up numerous times before: There was a 35-S 67 I was looking at (200$ish if I recall). 67 FB was 540 or so. This coin was as close as one could be FB without actually being a FB. I was at the dealer and we took it under the scope and the contact between the bands was less than a couple of um for about a distance across the fourth fasce on the left for the distance of not more than a few um.

Question to the above convoluted question: is FB purely a binary question,i.e., it either is or it isn't? Or, as the dealer proposed, given the closeness to it being a FB dime, that the proper valuation would be closer to the non FB price, in addition to 80% of (FB-not FB)?

Thanks.