mpizzurro said:
I can't find this is any of my brewing books or on line.
Your answer is in the MBAA Practical Handbook, Volume 2, Page 92.
Your range of 20 to 40% would just about cover it.
All fermenter freeboard (the term of fermenter headspace) is going to depend on gravity, pitching rate, fermentation temperature and yeast strain. The design or type of fermenter you use will depend on your brewing process. That's probably something you already knew.
I ferment under pressure and need only 8% headspace. The same gravity beer at atmosphere can fill 20% headspace and still yield 6% blowoff. So, the process change allows 10% more wort in the fermenter with no blowoff, for an increased yield of 16%.
High gravity ales are anything over 1.055, by brewing science standards, which is to say, nearly every craft and homebrew. A properly pitched and aerated ale, at just 4 F degrees above optimal fermentation temperature can create 50% more krausen. So, it's a sliding scale: higher gravity = more krausen.
On the other end of the scale, a properly pitched and fermented lager may only need 12 to 15% headspace at atmosphere and less than 5% under pressure.