The highest daily volatility ever was 15% and it's currently around 4%, according to https://bitvol.info/ . Two times the highest daily standard deviation ever doesn't seem so unreasonable.
For example, on Dec 7, 2013, volatility between the high and the low was 54.6%. [1] However, it was lower for the other days of all 30 day periods that include that day, and averaged out to under 15%.