You take a monthly average of the coin cost(you can do daily if you want but so far its not needed).  Let's say 100 low - 200 high.  Avg monthly mining is 150.  If you sell the coin for 120 then you take a -30 cap gain hit.  

I am 100% sure on this as I have been audited.  ASK YOUR ACCOUNTANT anyway.

Also they ruled that crypto IS property and not subject to wash rules.  I just did that to take a neg equity hit because prices are so low.  That means I sold all my coin at a loss and rebought them all a few min later.  Perfectly legal.  Wash rules apply to stocks etc, you have to wait 30 days to rebuy.