So 600M is the upper bound. Drew Houston owns roughly 25% of the company now, and he probably owned ~75% at when YC invested, for a dilution rate of 3. I'd say YC investment probably has dilution rate of 10, for the real amount of 60M. Not bad, but not Whatsapp scale either.
Dumb question: Why would YC get diluted so much more than DH? I've started to read up on venture financing and this doesn't match my mental model for any of the mechanisms I've learned about. (Like if anything I'd expect YC to have less dilution due to follow-on/pro rata investments, if they did any.)
I believe that when founders stock is fully vested, additional grants are an option to retain the founders beyond 4 years (or whatever the founders stock beating period is). 11 years as CEO likely led to additional grants. For a much bigger example, look at the additional grant Musk got this last week for Tesla.
Assuming no subsequent investments, dilution, selling of shares, etc.