You can use a computer, but it's not hard to calculate. One pound of liquid malt in one gallon of water is 1.036 gravity (36 PPG). It varies a bit with efficiency, but ballpark one pound of malt in one gallon of water should get you to about 1.025--1.029 gravity (25-29 PPG). Dividing, the two, 0.70--0.75 pounds LME should equal one pound of grain. Or, since it's what you're really after, 1.25--1.45 pounds of grain per pound of LME.
Just a caution as someone who does partial mashes a lot, when converting recipes always make sure you have enough diastatic power when you're done substituting to convert the rest of the mash! It's an easy mistake to make...