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...

I live in Seattle where the water is quite soft, which is nice because it's straightforward to hit a target water profile through adding brewing salts. I recently used the BrewersFriend.com mash chemistry and brewing water calculator, and chose the "Burton on Trent (historic)" target profile...