This link takes you to a page that just wants you to buy an app for the calculator for $5! I'm one who will not pay for apps if I don't have too. I did a bunch of googling and came up with an idea.
A web page says that thick yeast contains 4.5 billion cells per ml. So for 200 billion cells, you need 44 ml., or 1.48 oz of settled yeast. So I figure that I can use 1 oz of my yeast (135 mil cells) and do a good starter on my stir plate and should be fine, and have another oz to use for another batch.