DME is 45 points of gravity per lb per gallon; LME is 37. Most base grains are in the 36-38 range, so if you get 75% efficiency you'll get ~28 points. So
lbs of grain = lbs of DME * 45/27 or
lbs of grain = lbs of LME * 37/27
You'll also need to adjust the hops for the higher utilizaton of a full boil.
Many of the software tools (ProMash, BeerSmith) will do this for you.