What I do is take my AG efficiency, multiply it by the ag weight of the base malt in lbs and multiply that by 35 (max extraction for the grain).

So, as an example if I have 10 lbs of 2 row, and my efficiency is 75%, i figure I'll have 10x.75x35 = 262.5 gravity points.

Take that number and divide it by 45 (because extract packs 45 points per pound) and that will give you the number of pounds of extract you need to replace the 2 row

in this case, 262.5 / 45 = 5.83 lbs. of pale DME.

(If you are using LME, divide by 36 instead.)