If I have a pre-boil gravity of 1.054 and a post-boil gravity (OG) of 1.061, then by the following formula I have an approximate 12.5% rise in gravity due to boiling: (pre - 1.0) / (post - 1.0). Is it safe to assume that I also had a 12.5% drop in wort volume as a result of boiling? In other words is gravity as a function of evaporation losses a purely linear relationship? If so then I could probably get more accurate at knowing my overall brewhouse efficiency. My biggest current limitation is that I don't have a decent way to measure my "brew day complete" volume (stainless fermenter, no sight glass, no flow meter).