You won't need a starter for that little amount of beer (unless it's some monster high gravity). I'd just pitch the whole thing unless you plan on saving some to regrow later in which case I'd save a couple tablespoons worth. You might be moderately overpitched.
You can play around with the Mr Malty pitching rate calculator here and just look at the "Vials or packs needed without starter" after putting in your OG, 2 gallons and "born on date"
Assuming you have an Activator packet (not a propagator), you have ~100 billion cells. If you're pitching into 2 gallons of 1.050, you need 70 billion cells. Depending on how fresh the yeast is, the viability will impact what's in there. If you're packet is really fresh, it's going to have nearly all the 100 billion cells. If you go back to about Feb 1st (per Mr Malty) you're going to have about 70% viability ... or 70 billion cells; in which case you'll need the whole vial.