The idea of the viability date is to adjust yeast cell count for die off. So, a fresh smack pack has about 100billion cells. I just looked at the calculator and it is saying you should have about 10% of those viable. So, you're starting with 10billion cells.
Mr Malty will tell you the approximate number of cells you should pitch for a particular gravity beer. For a 1.050 beer, it says about 175billion cells (Note: most people generally say that you can direct pitch 1 pack for a 1.050 beer with good aeration, so YMMV)
The best bet is to take that 10billion cell starting point and use a yeast growth chart to figure out what you've got. The viability question isn't in question anymore but I haven't found MrMalty usable as the sole resource when you're doing a stepped starter. The yeast growth/starter charts are good because they work on the (# starting cells) vs (# liters of starter) = (# final cells). You can go from there then.
Jaz
