I vote to add water to dilute. Make sure you use one of those "top off water" calculators out there to get the right volune of water for your target gravity. If you don't you risk throwing your hops/bu ratio way out of whack.
In the future, with all-grain, always check your SG, and adjust before adding it to the fermenter. It's normally called "top off water", and it's legit.
I know for me, my boil off rate varies greatly from brew to brew. With the dry winter air up here I boil off nearly 50% more water then I do on a humid summer day. As you get more experience, you should be able to come closer to your target volumes, but it will still vary from time to time.