http://www.investopedia.com/terms/r/raroc.asp
If a government bond have 2% risk (volatility) but 4% return per year, the risk/reward ratio is 1:2, however bitcoin have 90% risk while have 4500% return, the risk/reward ratio is 1:50, 25 times higher than government bond
You can always reduce the risk of bitcoin investment to 2% by investing only 1/45 of the capital, achieving the same risk level as government bond, but much higher return at 100% per year. Even if bitcoin's value goes to zero, you lose maximum 1/45 of your capital
My point is that you can't satisfactorily gauge the Bitcoin risk at all (let alone set it to some specific value), since risk is a measure of deviation between annual returns of a financial instrument or investment, but you just don't have enough input to reach a conclusion that wouldn't be a wild guess. So how are going to accurately measure the risk of Bitcoin if it is only 6 years old and the majority of these years it languished in oblivion with its price next to nothing (yeah, I know it is painful to see)?
There is nothing which could be farther from the bell curve than Bitcoin volatility ("fat tails"), even if you take months instead of years to measure it