Where is the wort from? You should be able to guesstimate, you just don't want it too high or low. Check out
Mr. Malty...when I plugged in your numbers it looks like 1L of wort and the equivalent of 1 vial/ smack of commercial yeast is sufficient for the session beer you are brewing. Should be no problem. If you check the repitching from slurry tab, you will see the ml of slurry needed (what is on the bottom of your washed jars), in this case around 75ml. This is pretty easy to measure in a pyrex measuring cup or small mason jar.
Pitch 2 washed jars into your quart jar of wort. Make sure your starter container has enough headspace to allow for growth and krausen. If you have time before brewing, allow the starter to go for 36-48 hrs (it only takes about 24 hours to max growth but the yeast will build its glycogen reserves if you leave it be for a while after) then cold crash in the fridge to get the yeast to flocculate and settle out. Then decant the starter beer and pitch the yeast slurry only. If you are brewing in the next couple of days, pitch the yeast at high krausen, around 24 hours.