LME usually gives 38 points/lb (YMMV)
lbs grain to use = (lbs LME)*(38 pts/lb) / [(the malt's etract potential)*(your system efficiency)]
The first part, (lbs LME)*(38 pts/lb), is the total points from the LME. The second part, [(the malt's etract potential)*(your system efficiency)], is the extract yield per lb of grain on your system. The first part divided by the second part is the answer.
For example, your efficiency is 70% and you're using 6.6 lbs LME, and your malt has a potential of 30 points per lb:
6.6 lbs of LME x 38 points/lb = 250.8 points
lbs grain = 250.8/(.7*30) = 11.95 lbs grain.