If I can read his pictures right, that does not have the luster to be a 66. The nicks are not a problem, but the luster is. MS65
Looks like a PCGS slab to me, therefore, it's a completely random crapshoot anywhere between 63 and 69.
So far, and I can't claim to have made a scientific study of it, but most of the GTG threads I see seem to have been put up because the grade is a little hard to believe. Therefore, using "normal" analysis seems to me to be the wrong way to go. In other words, they're numismatic "sucker bets".