Being a simple guy, I used Mr. Malty to determine the approximate "Billions of cells" per ounce of slurry based on the typical thickness of the slurry. I use an average value of 60.5 billion cells per ounce. In my ingredients file, I set up "1 Packet" of slurry as follows,
<Name> = "Nottingham - slurry 60.25 Billion/oz" <Cells per pack> = "60.5" <Package date> = "Date of slurry harvest"
When designing a recipe I add "1 pack" of my slurry to the recipe. Looking at the starter tab I can see that a brew may need say 336.9 billion cells. Just below this I can see the aged value of viable cells per pack (or ounce) is 59.3 billion/oz. Then I can either use a calculator to divided 336.9/53.9=6.25 oz, or just look down the Starter tab to "Yeast packs to use if No Starter" in this case it indicates 6.
Since after much reading, I subscribe to the "More yeast makes better beer than Less yeast" theory, I pitch whatever is larger and easy to measure. Because I store my harvested slurries in canning jars, 8 oz is an easy measure. So I pitch at lest 8 ozs from the jar.
This method is not as mathematically precise as many described but I'm not convinced it has to be. It has been very reliable and easy for me to implement and given over 130 successful brews to date.