Honestly, for kit and extract brews, the boil volume is not that important. What is important is boil gravity, 1040 is the general concensus. What I do with my extract brews is do a 10L boil, and add the the rest of the extract (whatever is left over after I reach a boil gravity of said 1040) in the last 10-15 mins. This means you'll still get all you'r bittering as intended and the whole lot of the extract gets a bit of a boil to get it sanitised. Note that if any portion of the extract is liquid, and it's coming from an un-opened can, I don't boil it, it just gets added at flameout because it doesn't need to be sanitised as it's already been pasturised and sealed in a suitable container. In the end, with kits & extract, volume means sweet FA. All IMO and my 2c of course.