You only need to roll a dice 99 times to get a 256-bit number. Which gives you a bitcoin private key.
Given that 4 out of the 6 results add 2 bits and 2 out of the 6 results add 1 bit, then each dice roll gives on average ~1.66 bits. That's 256/1.66 = ~154 times. But, there's no reason to do this for a bitcoin private key and not for a seed, which will then generate infinite keys.
This looks very wrong.
Rolling a dice gives certainly more than 2 bits uncertainty, since 2 bits is one of 4 choices, while the dice is one in 6.
The correct way of calculating it is
log26 = 2.5849...Indeed 256 bits of uncertainty is very slightly more than 99 dice rolls.
You are loosing information when ignoring that there are 2 more choices in the first case, and 4 more in the second.
It is easy to do a check: write down the number 555..5 (99 times) in base 6, and convert it to hexadecimal (base 16).
The result is very close to 2
256F0BB8A1BBDE9163B9E053E8F918BF8E4D34034D7FFFFFFFFFFFFFFFFFFFFFFFF
One more roll makes it overflow (100 rolls)
5A4653CA673768565B41F775D6947D55CF3813D0FFFFFFFFFFFFFFFFFFFFFFFFF