I personally think that the "yeast experts" have been pulling the wool over our eyes. Yeast grow and grow and grow. The size of the starter, the inoculation rate are not really growth factors, but rather growth limiters. Yeast multiple by splitting into two cells and each cell will split about every 2-3 hours...in the right environment. Meaning you have to have enough food and oxygen. In the presence of oxygen (aerobic) yeast consume the starches and sugars and generate CO2 and water. Without oxygen (anaerobic) yeast will produce CO2 and alcohol. Alcohol is bad for yeast growth. The benefit of the stir plate is to keep the yeast off of the bottom, expel the CO2 and to bring in air (oxygen).
As Tom put it, the limit of growth is about a cell concentration of 220 M/ml or 220B/liter. At this concentration, the yeast will pollute their environment to the point of stopping their growth. All of the "yeast grow" is based on a very limit set of test conducted by Chris White. The growth limit of 6 is because that is where he stopped in tests and that is based on 100B cells pitched into 5 gallon wort to make beer. Beer is one of the worst environments if yeast growth is the goal. You can make good beer or you can grow lots of yeast, but not both.
I would suggest splitting your starting yeast into to two pitches. Store half in the refrigeration and make a starter with the other half. That half (100 B) will grow up to the limit of your starter size; if you use the 1.5L your will get about 300B+. And you should only take about a day, maybe a bit longer. Now switch, store the first starter and pitch your second batch. In the end you should have about 600 B cells; or pitch 1/3, grow to about 300B then add the 2/3 (130 B) saved for a total of 430B.
I am testing my theory now. I pitch a very small amount of yeast; about a match head size (~500M cells) into 500ml. I am letting the yeast settle out now to get an estimate of the growth. I am expecting the original 0.06ml of yeast to yield around 5-10 ml of yeast. That will be about 50 - 150 growth factor. Since yeast growth is exponential (ie 2n) that only represents 7 generations at the top end (27 = 128 x the original count.
The other thing you should consider is using oxygen in the wort and pitching a bit on the higher temperature side for 12 hours or so. In this environment the yeast will grow rapidly until they consume all of the oxygen. At this point lower the temperature to the normal lager temperature.
David