BYO has a good article on hop utilization and specific gravity of the wort. Wort SG of about 1.048 being optimum. This is a single reference. There may be other discussions out there that differ slightly.
The article also mentions boiling the hops in water may not be a good way to increase bitterness. Mostly due to the effects the pH of water has on extraction of other compounds in the hops. Hop oil utilization is greatest at a pH of 8.0. My well water is somewhere just over 7. Never had it tested to know what it really is at different times of the year. Even though a pH of 8.0 will extract the most goodness from the hops, alpha acids, it may also extract tannins adding a harsh flavor to the beer.
You can calculate how much extract to initially add for the volume being boiled to achieve a 1.048 wort.
Adding only the DME first, if used, rather than some DME and some LME will further enhance the final product being lighter in color.
Factor in the dilution of the IBUs that @porkchop points out in his post.