If that entire $100k was accrued that year. Presumably something was paid to get the BTC, which would decrease the gain commensurately.

I was going to mention this but it muddies the waters of the real issue.
Why is one cashing out $100k unless they are up like 5x or 10x?
The principal amount becomes irrelevant in these situations.
Considering our position within the 4-year cycle it makes sense to ignore the principal.