Without getting too scientific about it (which you can't, cause you don't know your cell counts/concentration or viability of your yeast), just make up 1.5 L of 1.040 wort and you'll be fine. This is assuming you're making the average 1.050-1.065ish beer.
Decant most of the liquid off before you pitch into the starter wort....but leave enough to allow you to swirl the yeast cake into a slurry, so you can pour it out of the jar.
Someone else may chime in with another approach, but this has worked for me.
Last edited by BuzzCraft; 02-17-2010 at 05:02 PM.