My thoughts were that it would be useful for AG or extract brewing. I was reading this article about hitting your OG. http://www.hoptomology.com/2012/02/14/h ... l-gravity/. Where he missed is OG based on a given volume after the mash. From the calculations, he could determine if he needs to boil longer to raise the OG or add more fermentable materials.
That lead me into a discussion about taking a gravity reading after you cooled your wort and before you put in fermenter bucket. If I wanted a 5 gal batch, and I started with 6 gal boil, my boil off might be 2 gal. I will get a better gravity reading from the wort before adding 1 gal of water back. If my BG ended up being on the low side, I might not neet to add a full gal So if I take the reading and then caculate what volume I need to add to hit my OG I could add that to fermenter bucket and do not need to take another reading because I would be at my OG from the given calculation. specific gravity x volume = gravity units
After reading the article, I found the couple useful calculators related to adjusting your volume based on your starting OG and your target OG and your current volume. http://www.brewheads.com/newvolume.php
And then another calculator will tell you what your OG for a target volume based on gravity reading and current volume and Your target volume. http://www.brewheads.com/newsg.php
Both I guess come from the same formula. specific gravity x volume = gravity units
Hope this makes some sense about what I'm trying to say. I'm just getting into home brewing so I may be off target.