This supposes boiling down to a to a target volume. But most people boil for "X" minutes, and hitting a target volume is just a function of dialing in the process (total water in this case).
damn, i just sparge until i have a full kettle...then boil until i guess i got about an hour left, then toss in some hops....usually my boil take over two hours to even look like i only have 45 min, hour left on it....when it looks close to final volume i start checking with a nothced stick....
(i know probably not appropriate for me to post in a thread with "advanced question" in the title, lol)