As I have mentioned many times on this forum, yeast cultures are kind of like nuclear weapons in that one only needs to be within a reasonable distance from one's target in order to accomplish the task. The difference in propagation time between a 1L starter and a 2L start is one replication period (approximately 90 minutes); hence, very little is gained by pitching 2L starter. Nothing is gained by pitching a 2L starter that has reached quiescence over pitching a 1L starter at high krausen.
In my humble opinion, modern home brewing yeast pitching dogma is doing more harm than good. My discussions with the brewers at White Labs only confirmed this belief. I have pitched as little as 3 billion cells per liter of wort and as much as 20 billion cells per liter of wort. Both pitching rates created good beer. The difference between the two extremes is that the 3 billion cells per liter rate allowed the yeast to express its unique character whereas the 20 billion cell pitching rate produced a more generic flavored beer. Granted, one has to pitch more cells per liter with high gravity beers, but that's only because high osmotic pressure coupled high ethanol levels take their toll on yeast cells, and it is more difficult to dissolve O2 in high gravity wort than it is in lower gravity wort.
Those who have tried my starter method have reported improved fermentation characteristics. A large part of that improvement is pitching at high krausen. A starter made with extra light DME (e.g., Briess Pilsen) that is shaken until the media is mostly foam at the beginning of fermentation, and not stirred will be very neutral in flavor. A 5% increase in final boil gravity will allow for a dilution rate of 1L per 5 gallons. I guarantee that pitching at high krausen instead of waiting until quiescence has been reached will cut your starter volume in half because the cells have not depleted their ergosterol and unsaturated fatty acid reserves, nor have they undergone the survival-related morphological changes that occur at the end of fermentation. A 1L starter that is pitched at high krausen will usually double cell count-wise by the time that a 2L starter that is pitched after quiescence had been reached exits the lag phase. The net O2 load from the 1L starter will also be lower because one half of the cells at this point came into the game with non-depleted ergosterol and UFA reserves. This difference results in a healthier fermentation.
If that information is not enough to convince you, the difference between pitching 200 billion cells and 400 billion cells is insignificant when pitching normal gravity wort. The maximum cell density for 1L of wort is approximately 200 billion cells. This limit is controlled by cell size. The maximum cell density for a 5-gallon batch is 19 * 200 billion = 3.8 trillion cells; hence, neither pitching rate will saturate a 5-gallon batch of wort without significant growth. As the cell count grows at a rate of 2n, where n equals elapsed clock time divided by the number of minutes in a replication period (around 90 minutes under ideal conditions), the minimum number of doubling periods that are required for each starter size to saturate the wort are as follows:
four_hundred_billion_cell_starter_replication_periods = log(3,800 / 400) / log(2) = log(9.5) / log(2) = ~4 (arithmetic ceiling taken)
two_hundred_billion_cell_starter_replication_periods = log(3,800 / 200) / log(2) = log(19) / log(2) = ~5 (arithmetic ceiling taken)
