MOD17 UPDATE! New formula tested by Janne and Sharpedge:
critical healing chance = 0.4 x (critical strike / power) = critical strike / (0.25 x power)
I also recommend to check out Janne's page about healing mechanics! https://jannenw.info/pages/mechanics16/heals
My old post from Mod16:
Since the AMA we know that crit healing is based on a ratio between critical strike and power. So I did some tests to find a formula. I was using different ratios while standing in the lava in Mount Hotenow and healing myself with Bastion of Health.tl;dr: I think the formula is approx:
crit chance = 0.333 x (critical strike / power) = critical strike / (3 x power)cap is 50%
Tests to check the cap:
9k Power and 22k Critical Strike. Result: 222/417 crit heals (53%).
50k Power and 120k Crit. Result: 100/200 (50%).
Conclusion: seems to be capped at 50%. Would make sense because DPS crit chance is also capped at 50%.
Tests to check the relation between Power and Critical Strike:
||Chance / Ratio
As you can see, the "Chance / Ratio" is always around 0.3. So my approximation seems to be good. But all tests were done under the same conditions. For example, not considered in my tests are: scaling, character level, zone level (counter stats) and so on. Moreover, I didn't test for a minimum crit chance.
It could also be 0.25 * (crit / power) + 0.05?
I hope this is useful for someone.