'Theoretical maximum' as total ethanol potential calculated by mass.. 13lbs of pure glucose is 1gal pure EtOH. Of course, fermentation isn't straight material balance, so we tend to see you only actually get about half of that(~26lbs.)
With basic whiskey yeast in a single malt mash(2:1 lb grain/gal water ratio, no added enzymes, grist size of about <600um, and 4 step mashing protocol), 1gal 100% EtOH per ~30lbs is pretty standard. That's right on with Cutty's numbers.