Thanks for the reply -- the valuation method I mentioned was obviously too crude, so I was hoping someone would call me out on it. I don't know much about finance but I'm actively trying to teach myself. Anyway, it would be good to find a more reliable way to quantify a company's long-term, potentially risky growth opportunities.
There's a million models, but there isn't going to be one that's universally correct. The problem is that the inputs are generally too subjective and too noisy in any model that includes future earnings (there are some conservative ones that are based only only on past performance, but they will never match with an IPO valuation.)
A great place to start is this book: Value Investing: From Graham to Buffett and Beyond ... by Bruce C. N. Greenwald ... most folks will say it's too conservative but it's a starting place.
This calculator: http://www.ft.com/cms/s/2/8a21debe-944e-11e1-bb47-00144feab4... was kinda useful, but a more detailed model would be interesting.