Below is an explanation of the method I use to step up starters when the flask volume is limited:
Assume I'm making an 11 gallon batch of 1.050 lager and I have 1 vial of White Labs yeast. Mr. Malty tells me I need:
Cells needed = 771 billion
To get 771 billion cells, Mr. Malty recommends pitching 2 vials into a 6.8L stirplate starter(SPS), or 3 vials into a 4L SPS, or 4 vials into a 2.75L SPS. My largest flask is 4L, so I'm limited to about 3-3.5L of starter. So, the only feasible option is pitching 4 vials into 2.75L. I only have 1 vial of yeast, so that doesn't help me BUT it does tell me that if I pitch 400 billion cells into a 2.75L starter, I will get my desired cell count. So, now I know I need to step my 1 vial up to 4 vials/400 billion cells.
To determine how to get 400 billion cells you just need to enter a theoretical batch that requires 400 billion cells. For example, Lager/1.050/5.7gal. It doesn't matter what you enter, the starter will be the same if the cells required is the same. Try Ale/1.090/6.6gal, the starter is identical. It recommends either 1 vials into a 3.7L SPS, or 2 vials into a 1.5L SPS.
The 3.7L volume is still a little too much for my flask, so I go through the same exercise again. I want to step up my 1 vial to 2 vials/200 billions cells. I enter Lager/1.050/2.9gal. Mr. Malty tells me I can get 200 billion cells with 1 vial in a 1L SPS.
So, this is what I do:
1.) Pitch 1 vial into 1 L for a yield of ~200 billion cells.
2.) Chill, decant.
3.) Add 1.5L of sterile wort to decanted yeast, yielding ~400 billion cells.
4.) Chill, decant.
5.) Add 2.75L of sterile wort to decanted yeast, yielding ~770 billion cells.
6.) Chill, decant.
7.) Pitch to 11 gallons of 1.050 wort.
