How did this turn out for you? I'm doing the exact same thing and am glad I found your post. My first full boil was an IPA. I'm using a 7.5 gallon kettle and a propane burner (standard Turkey Fryer). I figured out that I need to start with 6.25 gallons and after a 60 minute boil I'm left with 5 gallons. I steeped my specialty grains at 160 for 30 minutes in the full 6.25 gallons of water. I noticed when I went to bottle it, that it had an oily appearance.
After reading your post from BYO magazine I'm guessing that these were tannins, and that I should have steeped them in 3 quarts per pound of grain, THEN added that to my brew kettle, and then added water to top it off to 6.25 gallons before starting the full boil. I had never read or heard about this before. I also bounced the grain bag up and down constantly for 30 minutes while steeping at 160, so maybe I shouldn't do that either. Maybe that also extracts too many tannins and I should just leave it alone?
If you see this, let me know how it went for you if you don't mind! After 5 batches I'm getting frustrated with extract and am considering going All Grain on my next batch. But I'd really like to make a few good extract batches and know how to do it before making the move. I just haven't been happy with any of them so far.