Okay I looked it up, each Yeast weighs ~8x10^-11 Grams so 100 billion yeast will weigh 8 grams. Water weighs 1 gram per 1 milliliter, so if you weigh 100ml of your slurry and it weighs 108 grams you have 100 billion yeast in your suspension or 1 billion yeast/ml.You can do a little math and make a simple formula
((weight in Grams of 100ml of slurry)-100g)*(100 billion yeast/8g)=total yeast in 100ml of slurry.
Depending on how well you rinsed your yeast you might want to adjust this number down but 10-15% to account for non yeast solids in the slurry.
Hope that helps.