Estimating Remaining Finish

[Arizona Slim]

Is there a guidline for estimating the remaining finish on a firearm available? I know this is subjective, the seller will generaly estimate the finish at a higher percentage then the buyer . I always assumed that the percentage was an estimate of how much, if any, bare metal was actually exposed. Does the number also include an estimate of how thin the finish is perceived to be? The reason I ask is that the last couple of firearms I purchased through a major auction house the firearms were estimated to be 70% on one and 80% on the other, when I got them there was actually very little bare metal showing. There are several books on the market to help coin collectors estimate the grade of a coin, such as "Photograde", anything available for gun collectors?

Lon