This is a bizarre, and absolutely meaningless statement. 13.86 per cent?
Although I sympathize and understand Mr. Koehler's desire to 'get a handle' on this craft and the details of its practice, there is simply no way that such a factor as the moisture in the paper can be quantified to a single value ... and certainly not to two decimal places.
The amount of water present is determined by a number of factors: the size of the area to be printed, the amount of moisture present in the pigment, the way that the paper has been sized, and most of all, by the type of impression that the printer desires to produce. The paper must sometimes be 'quite' wet ... sometimes 'quite' dry.
I do not want to leave the opposite impression - that this is a 'mystical' factor that cannot be comprehended by us mere mortals. Mr. Koehler got it right in the sentence at the top of the paragraph - 'the proper degree being determined by the judgment of the printer'. He should have left it there.