You will want to use a pale malt as your base, either 2 row or maybe British Pale like Maris otter. Take your 6.6lbs of LME x about .75 like duboman said. Then, your color, roast flavor etc will come from your specialty grains. Generally with a stout, that is roast barley, chocolate malt and some crystal. You're probably looking at about 75% base malt, about 7-10% caramel, 7-10% choco and 3% roast barley. I'd suggest you read 'Designing Great Beers' by Ray Daniels. It has a wealth of info on how to calculate gravities, converting from extract to grain recipes and how to build beers in just about all the styles, good book to have around. By playing around with the percentage numbers, you can adjust sweetness, dryness or roast flavor. I agree with the above post as well, Beersmith is a great tool to have for creating and tweaking recipes. HTH.