That's it! I took the reading from the wort with only 2.5 gallons of water mixed in (initial boil). Should I bother taking off plug and airlock to take again now that the full 5 gallons is in the carboy?
If you have a hydrometer thief (wine thief) and want to, go for it. But truthfully, if you topped off to the right volume, you hit your gravity since you're using extract. Be sure to aerate thoroughly, let it ferment, and take an accurate final gravity reading, so you can gauge attenuation.