Sum1Stu: The amount of hop bitterness extraction depends on the gravity of the wort.
Papazian's The Complete Joy of Homebrewing has a Hop Utilization chart which states you get the maximum bitterness extraction (30%) when using 1 gal of water with 1 lb DME and 1 oz of hops...which should equal a 1.040 gravity. Brew Your Own mag uses 1.045 for the same. Why the difference? I don't know?
I've been using Papazian's calculations since 1994 and will continue to do so because I am comfortable with it.
So, if you boil 2 gals then you should only be adding in 2 lbs of DME or its equivilant grain (3lbs, 8 oz) or LME (2 lbs, 10 oz).
If you'd like a copy of a chart I made (using the calculation of others) I'll gladly send you a copy. Email me at firstname.lastname@example.org
I usually boil for 45 mins, remove the pot from the heat and add in the remaining DME and steep it for 15 mins.
Top off water is usually gal jugs of PUR filtered tap water in the freezer for 4-5 hours prior to brewing.