1 lb of base grain = .8 lb liquid extract = .6 lb dry extract.
I use Palmer's countertop PM method from BYO using 4 lbs of grain in a 2 gallon cooler. I mash all of my specialty malts with as much base malt as I can squeeze in to get to 4 lbs. I then figure out how much extract the base grains are worth, and subtract that from the total extract amount in the recipe.
Using BeerAlchemy, set your procedure to the partial mash setting for an acurate assesment of your recipe.