When I did extract brews, I cooled my wort, then poured it through a strainer into my fermenter. The strainer caught all the coagulated gunk from the hot and cold breaks, so I never lost more than a quart through sediment in the primary. Now I brew all grain and use a counter flow wort chiller. All of the cold break piles up in the bottom of my fermenter, so I usuall shoot for about 5.5 gallons post-boil.
If you have extra malt extract sitting around, you could add a few ounces of that and an extar quart of water. Really though, the loss of volume is small and doesn't affect the quality beer, so why mess with pre-formulated recipes.