[TippetsTX]

So not everyone agrees it is 'official', but the old Fantasy Master's Codex has the breakdown you are looking for. TBH, I prefer it since it reinforces the idea that as task difficulty increases (adding more dice), your odds of a critical success will go down.

On 4 dice...
4 = critical success/triple damage
5 = critical success/double damage
6 or less = auto-hit/success
20 or greater = auto-miss/failure
21-22 = critical failure/drop weapon
23-24 = critical failure/break weapon

On 5 dice...
5 = critical success/triple damage
6 = critical success/double damage
7 or less = auto-hit/success
24 or greater = auto-miss/failure
26-27 = critical failure/drop weapon
28-30 = critical failure/break weapon

On 6 dice...
6 = critical success/triple damage
7 = critical success/double damage
8 or less = auto-hit/success
28 or greater = auto-miss/failure
31-33 = critical failure/drop weapon
34-36 = critical failure/break weapon