Clearing up some definitions… Adjuncts are ingredients that contribute starches but lack the enzymes to convert those starches into sugars in a mash. Rice, corn, oats, unmalted barley, etc. When using adjuncts, you need to make sure there are enough enzymes coming from the malt to convert both itself and the adjuncts. This is called "diastatic power".
20% is a pretty conservative recommendation, though maybe it was accurate in the 70s. With modern 2-row pale malts being so enzymatically active, you can use at least 60% adjuncts in the mash without issue. (Well, you might have trouble lautering, but you'll be fine on the enzymes.)
Chocolate wouldn't be an adjunct. I guess you could think of chocolate malt that way, but we generally just call that kind of thing a "specialty malt" or "character malt" or something like that. Specialty malts would never total 60% of a grain bill (and really shouldn't be more than 20% or so in most styles), so diastatic power just isn't an issue when working with a normal recipe.
To actually answer the question, home brewers usually work with extract in "point-gallons" and the potential extract of ingredients in "point-gallons per pound", which is almost never said/written correctly and generally abbreviated as ppg. If you look at the units, you can see that multiplying the pounds of an ingredient by its potential extract gives total extract in point-gallons. Divide that by the total extract of all the ingredients and you have the percentage of fermentables.
Or, much easier, divide the weight of the ingredient in question by the total weight of the grain bill to get pretty close.
Topic: How do you measure percentages of fermentable sugars? (Read 2101 times)
