I tried a experiment on my last brew. After pitching my starter I poured a litre of 1.020 wort in to see if I could build off of the yeast that what was stuck to the sides and bottom. It was a better success than I thought. After that decanting the 1 litre I did a normal 2 liter starter and now I have a good 1/2" of yeast in the bottom of my 2 litre flask.
Any idea how much I have? I'm going to put it in a sanitized mason jar to keep until I need more.