There wasn't much more you could do there. The stout will definitely be a bit sweeter than you wanted, but who knows, it could be the best thing you've ever tasted.
In the future, especially with big beers like this, pitch the proper amount (likely 3 packets in this), rehydrate the yeast in 30ºC water 30 minutes before pitching, and aerate the wort with bottled O2. This will be your best chance of getting the proper attenuation.