Liquid malt extract is much harder to use since it is very thick and will stick to everything, therefor it is harder to measure for a particular gravity starter. John Palmer recommends 1/2 cup DME (easy to measure) with 1/2 Quart water.
I would give 1/3 cup of LME to 3 cups of water. You'll want to heat the water and use it to rinse your measuring cup to get all of the LME out. That should give you a starter of 1.040.
I keep a few pounds of Dried Malt Extract on hand all the time just for starters.
I am actually wondering the same thing. All I have is LME and I planned on brewing tomorrow. The only LME I have is from the kit I bought. Is 3 cups to 1/3 cup LME good and is it ok to use the LME I have from my kit ?