Originally Posted by Matt Up North
if you want to water down your beer then yes. If you are ever worried about your volume, you have to think of it prior to the end of boiling as all grain so that you can add DME. You have 4 gallons of drinkable beer, work on your process to get it so that you are able to hit your volume marks just right.
Not necessarily. If you boil too much and come up short on volume, chances are you overshot your gravity too. You'll have 4 G of a higher ABV beer. You can water it down to get 5G of what you expected or leave. If you're worried about the O2 in the water, boil it for20-30 minutes to bring out the O2, the let it cool and siphon into secondary, then rack beer on top to mix.