The percentage of your holdings to put in a Roth IRA depends on your age and how likely you are to want to access that money before you reach retirement age.

If you are young, you probably only need a smallish number of bitcoins in the IRA.  By the time you reach retirement age, their value will probably either have grown a whole lot, or they won't be worth much at all.  So a small investment in a Roth IRA today either means a very comfortable retirement or a small loss.  At some point making a larger investment multiplies the risk without significantly improving the outcome.  Your retirement won't be that much different if you're worth $100 million instead of just $10 million.

However, the rules of the Roth IRA allow unlimited, tax-free withdrawals once you reach age 59 1/2 (as long as you have had any Roth IRA account for at least 5 years).  So if you're already that age, or are willing to wait until that age to touch the money, you would want to put most of your coins into the Roth and avoid all those capital gains taxes!