Some beers turn out better with a little sugar. Belgians work well with a healthy sugar addition where you want to boost ABV while keeping the body light. The sugar can be 15-20% of fermentables in some cases.
If you wanted to make a modest boost to other styles, I'd say a sugar addition of 5% of fermentables will be almost undetectable. You could go up to 10%, but this might be noticeable. I have not had issues with stalled ferments, but I always pitch plenty of yeast and use yeast nutrient also.
Whoa, that's a lot of grain. 12 lbs of grain gets me nearly 7% on a 5 gallon batch of IPA, which is around 77% efficiency. Lower gravity brews get me into the high 80's.
First thing I would look at is your crush. Then mash conditions (consistent temps, good stir w/ no dough balls, proper water). As long as you end up with 10 gallons (and not over), I don't see how your sparge volume would be the culprit. There are lots of threads here on lower efficiency. If you need to bump up your ABV a tad, you can always add 5-7% sugar in a pinch. Or better yet keep some DME on hand to boost your ABV up to targets until you figure out your efficiency issues.
He's doing 10 gallon batches.
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