Someone help me with the math here.

If I'm brewing a 5 gallon batch and want an OG of, for example. 1.060, how much should I expect my OG to drop if I wind up with six gallons of wort isntead of 5?

I guess the basic question here is, if you have wort that's 1.060, what would the reading drop to if you added a gallon of water?