Sepping up yeast starters

I have a question about stepping up yeast from a single vial to the equivalent of 11 vials.  Because there is an upper limit on yeast growth rates I need a three step process.  For my purposes I chose growth rates of 2.5 for the first step, 2.2 for the second step and 2.0 for the third step.  This results in an overall growth rate of 11 without pushing the growth rates for any individual step.  According to dieseldrafts’ spreadsheet I need a total starter size of 4400 mL, which will give me 1100 billion cells.  With a growth rate of 2.5 I need an initial step size of 1000 mL.  With a growth rate of 2.2 , I need a second step size of 2200 mL and with a growth rate of 2.0, I need a final step size of 4400 mL.  My question is this.  Can I add 1200 mL of fresh wort after the first step has fermented out and then 2200 mL to the second step after it has fermented out which will result in a total of 4400 mL or do I need 2200 mL of fresh wort for the second step plus 4400 mL of fresh wort for the final step?  I use a stirplate.  Does it make a difference if I decant off the spent wort after each step?  With such large starters I decant off the spent wort from the final step to avoid reducing the gravity of my beers and the stuff taste bad because I am growing yeast not trying to make beer.

I crash cool between each step and decant off of the slurry. Recharge it with fresh wort in the 1.020 - 1.030 range which helps create cells, not alcohol. You will be making beer later.
The stir plate will help immensely, as does the large volume container you appear to have.
I personally think folks get too wrapped up around the "Mr Malty's" of the world instead of just trying to get as much yeast out of the particular system they are using. If all you had was a 1 liter flask and could perform the three step growth process, you will still make beer. Even if it doesn't match the calculator. Yours appears to be larger than most and should provide you with good results for high gravity wort. 

Instead of stepping up to almost a 4.5 L starter (and I'm not sure even that would get you the cell count you are looking for), you might consider doing multiple smaller starters. As you noted there is a yeast growth ceiling and pitching rate has a huge impact on yeast growth--you run into diminishing returns rather quickly. According to White and Zainasheff in "Yeast," 1 vial in a 2L starter will double your count to 200 billion. These rates are assuming a simple starter w/ no stir plate, stir plate will double the growth to 300 billion.

If you took one vial and pitched into a 2L starter, got your growth, allow the yeast to rest after exponential phase to build glycogen reserves, chilled, then decanted the "starter beer" and pitched that yeast into a fresh starter you would basically be pitching 3 vials into a 3L starter which would yield about 6-700 billion cells. At this point it would be better to separate a portion of the yeast and build a smaller starter than to step up the entire thing into one big starter.

I hope this makes sense. I prefer to keep the starter size smaller and more managable. Personally, rather than build such a huge starter, I pitch a starter propogated from a lab culture into a low grav, lightly hopped brew and then harvest and wash the cake. I end up with about the count you are looking for if not more, I get a batch of beer, and I acclimate the lab culture to brewing!!! Many brewers feel like their lab cultures don't really produce the best brews until around the 3rd generation.

I highly recommend "Yeast," as it is loaded with useful information regarding yeast propogation and storage. Happy brewing!

I have been making 2L starters for my last couple three starters  I have had active fermentation in a 6 gallon batch in under 8 hours.  Using a 2L flask & stir plate