Thanks for the tips.
I did not add all of the extract the recipe called for because I ran out of room in my pot (new 5 gal. pot is on the way), and in addition to this, I ended up with more water than the recipe called for (next time, I won't add the pre-boiled water to the fermenter until after the wort has been added so I can make the batch precisely the correct size). And so, I think the batch could stand a bit more extract.
Right now the airlock is bubbling away. Should I wait until fermentation slows down before I add more wort? Or is now the right time to do it?