Yeah, I've read and heard that maximum bitterness extraction is achieved in a wort with a gravity of 1.030. So you can use your favorite brewing software to figure out how much extract to add to your volume of water to get to 1.030. Add that amount as your wort comes to a boil, then add bittering hops, and add the remaining extract about 15 minutes before the end of the boil. For 5 gallons, I think about 3.5 pounds should give roughly 1.030.
Using a brewing software to calculate this is a good idea, since then you can estimate the IBUs, taking gravity, hop alpha acids, amount of hops, boil time, water volume, moon phase and whatnot into account.