The highest I'm aware of is around 8.5% in CA.
California state tax rates are 1%, 2%, 4%, 6%, 8%, 9.3%, 10.3%, 11.3% and 12.3%.
Based on my own experience, if you send the IRS 20% of the gain, then come tax filing time you'll be fine.
You'll be fine if you just do as @Critter-3 posted as corrected by @Mike9241. The idea is not to get a penalty.
