If it were me, here is what I would do. I would do a 2L starter with the single smack pack. Let the starter ferment completely out. I would then cold crash the starter in the fridge for 24-48 hours. I would then make another 2L starter. Decant the fermented beer off the yeast from the first starter and pitch this yeast into the new second 2L starter. I would then pitch this starter into your brew when it reaches high krausen. This won't get you to the 300 billion cells, but it will get you closer than a single smack pack with a single starter.
I also don't think the Mr. Malty calculator over does everything. However, it should be used as a guide and not something set in stone. I have noticed a drastic increase of the quality of my brews when using proper pitching rates and proper fermentation temperatures. There is a lot more to fermentation than just throwing a few yeast cells into wort. Yes, they will ferment, they will multiply, they will turn your wort into beer. Proper pitching rates and temperature just lead to a healthier higher quality fermentation.
Edit: after looking at the MrMalty calculator, you might be able to get away with a single starter with one vial. It looks like you are border line of needing one or two vials with an SG of 1.088. If you are using a stir plate, you could probably getaway with using a single vial 2L starter.
