I worry more about hitting my OG than having a certain volume, and this can be figured out through straight algebra. Measured Gravity x Volume = Desired Gravity x Volume.
For example, say you wanted an IPA with an OG of 1.065, but you wound up with two gallons at 1.075. To hit your intended OG (1.065), you would need to add .31 gallons.
Here's the math: 75 (from 1.075) x 2 (gallons) = 65 (from 1.065) x Unkown Volume (gallons). 150 = 65 x Unknown volume. 2.31 = Unknown volume.
Since you already have two gallons, adding .31 gallons of boiled and cooled water would give you 2.31 gallons of 1.065 wort.
In regards to OG, it impacts where your FG winds up, and your FG impacts the overall balance of the beer.