Your beer is fine. You underpitched, but it will still taste great. I read studies on underpitching vs. proper pitching and the results were not as cut and dry as you would think.
That said, a starter is a wise decision. Especially for a 1.07 beer. Usually a beer over 1.06 you would want a starter or two vials/smack packs. But your beer has fermented out so I don't see a reason to get bummed out if you didn't.
Use this site http://www.mrmalty.com/calc/calc.html
to determine what you need.