You can generally draw wort until the SG drops to about 1.010. It's kind of a pain to measure if you don't have a refractometer, but you can always batch sparge like Denny does, that should help avoid any tannin extraction. Follow the link in his sig.
For a 90 minute boil you should just add more as you go, either wort or water. Or you can add water to the fermenter to top it up. Or you can adjust your recipe to expect less in the fermenter.
Re: the cold break, I use a CFC so the whole cold break ends up in my carboys. No problem. YMMV