Recommended grain bill per volumes?

[30xs]

I am getting ready to try my first AG recipe, Booners for the lower cost to failure ratio. I was wondering if there was a thread that I haven't found that would give someone an idea of fermenter volume to grain ratios? A link would be wonderful, or lets build one here for others to reference. Since doing a no cook I don't want to have too much grain in and not be able to get my water in. I ground a 5 gallon bucket of corn and it weighed out to about 26 pounds. My fermenting barrel is 20 gallon. Using a 2 pound per gallon that would be 5 gallon (#26) plus 13 gallon of water leaving 2 gallons of headspace, correct me if there are variables that I'm missing or unaware of such as grain swelling from hydration.