The calculator looks like an excellent resource.
This now leads me to a second question. In my first batch I had added 2lbs of sugar for a 6 gallon batch. So, I'm expecting a low ABV for my first batch. For my second LME English Bitter kit, I picked up 2lbs of DME rather than going with sugar after beginning to read the Joy of Homebrewing. It seems like my 2lbs of DME is not going to be enough to get my alcohol content up in the English Bitter. Am I right? Should I kick in a little bit of sugar in addition to the DME?