I probably am over-sparging. My FGs tend to be in the 1.020 range where the OG is around 1.120 (or so).
I should actually clarify something. I boil down to 4 gallons then pitch a 1 gallon yeast starter made with a pound of LME (calculated into the recipe, not additional).
Also, i just read the article on producing BIG beers and that helped a lot, and i do use up to 15% sugar in most of my beers, which is probably why i get such attenuation.
Next time i will sparge a bit less, aiming for the 1 1/2x recommended here. You know, and you help clarify something for me: preboil gravity. I always thought it was taken after you sparged, so it never calculated correctly. Now i know why: you take it of the mash before you mix it with the sparged water. oh well.