Start a company today, and if you're lucky, you'll sell it no sooner than 5 years. In that case, you'll most likely qualify for QSBS, which means you'll pay an effective rate of 0%. Only available for C-Corps. You're welcome.
The % deduction you can take is entirely dependent on the year and sometimes month of when your stock is issued. For example we had C Corp stock issued in February 2010 so our FEDERAL deduction was 75% NOT 100% which it would have been if it was issued in November of the same year (Blah), also California no longer has the QSBS deduction so you still pay the 8%+ here.