How much of the 125 ml is yeast?

On the smack packs, it says that it contains 125 ml of liquid yeast and nutrients.  How much of that 125 ml is liquid yeast?  I am starting to harvest yeast from fermenting batches, and I would like to start making guestimates on how much slurry should go into a starter when it is time to brew again.  Thanks for any help.

Have you checked the slurry tab on the MrMalty calculator? Basically 1 mL of fresh, packed slurry (after a couple days in the fridge) will be 3-4 billion cells.

And Wyeast claims 100 Billion cells in a smack pack.  So, using 4 Billion cells in a ml, a smack pack would have about 25 ml of actual yeast, right?