The calc from Mr. Malty tells me that with a harvest date of early december, I'd need 387 billion cells, or 614 ml of yeast.
That number of cells you need is independant of yeast viability. You'll need 387B cells regardless of viability. Yeast viability just tells the calculator about how many cells you actually start with.
If you click the 'Repitching from slurry' tab you'll see that a thin slurry is approx. 1B cells per ml and a thick slurry is 4.5B cells per mL. You can approximate your starting yeast count by multiplying your slurry volume (in mL) by 4.5B (this is settled yeast in a jar).
When I put December 1st in the liquid yeast 'Production Date' box, it says the viabilty is 67%. So multiply the result you got above by .67 and that should tell you about how much yeast you are starting with. It's a gross approximation but it's about all we can do.
In the liquid yeast tab, the yeast viabilty percent is actually telling you how many billions of yeast cells you are starting with (this is because a fresh vial/Activator pack is ~100B cells, so 67% is 67B cells and 150% is 150B cells). Yes, you can enter a percentage greater than 100.
So now you can just enter the billions of yeast cells in the 'viability' box, make sure the '# vials or packs needed with starter' is 1 (if not, slide the Growth Factor bar all the way to the right), enter all the other info for your beer, and it should tell you how big of a starter you need.
Wow, that got long and it looks way more complicated than it really is.
EDIT: If you didn't get all that, if you could post the approximate volume of your yeast slurry in mL (that's settled yeast in a jar) then I'll go through the entire example with your numbers.