I've seen some games solve this problem with the concept of "double-crits", which I like.
For example, let's say a crit deals +60% damage based on their crit rating and they have a 30% chance to crit.
Then, something buffs them with +100% chance to crit, their new crit chance is effectively 130%, but that becomes a 30% chance to double-crit for +120% damage with the remaining 70% chance guaranteed to crit for +60% damage.
