Yeah you really don't know what you have from a slurry. I have used the Mr. Malty slurry calculator and the beer came out fine, but recently found it's estimations are way off. The "thick to thin" slider doesn't even cover the entire range of cell density of my slurries, and I would have called all of mine "thick" doing a count. Also the viability from havest can range anywhere from 30%-90% on the day of harvest. The good thing is that it stays fairly viable once it is in the fridge.
bottom line, assume your cell count could easily be off by a factor of four when entering numbers into the starter calculator. The easy way to get an estimate is to assume 100 billion cells per liter and don't bother with the calculators.