In regards to the original question of how big a starter to make from a harvested slurry.
I had the same question on another forum and this is the answer I was given.
I can't take credit for this.
Let's say I'm making a 1.060 ale, pitching 5.5 gallons. I have a jar of reharvested slurry that is exactly four weeks old from today, and the yeast is very compacted into a 50mL puck. I go to Mr Malty, set the parameters of my beer, set the age of the yeast, go to the "Reharvesting" tab, set the Yeast Concentration to 4.0, set the Non Yeast Percentage to 15.
Mr Malty says that I need 229billion healthy yeast cells, which would be 134mL of my yeast in its current state. I have 50mL, or 37% of what I need (50/134). If you multiply 37% by 229billion cells, you find that I have about 84billion healthy cells.
Here's where things get fuzzy. As I understand it, the Mr. Malty calculator works such that a vial of yeast with 100% viability has 100billion cells. So if I have 84 billion cells, this is the equivalent of having one vial of yeast at 84% viability, right? So then I flip to the Liquid Yeast tab, manually set my viability to 84%, set the Growth Factor so that it says I need 1 vial. Doing this, it says I need to make a 1.23L starter with a stir plate.