Okey, as this mechanic isn't that easy to math out, I wrote a little programm to simulate it.

The result, after 10 million simulated Soothing Mist channels (at 9 ticks each):

Average Chance to gain Chi: 34.02%

Chi gained: 0. Chance: 0.0%

Chi gained: 1. Chance: 1.4%

Chi gained: 2. Chance: 25.2%

Chi gained: 3. Chance: 45.1%

Chi gained: 4. Chance: 22.9%

Chi gained: 5. Chance: 4.9%

Chi gained: 6. Chance: 0.5%

Chi gained: 7. Chance: 0.03%

Chi gained: 8. Chance: 0.0008%

Chi gained: 9. Chance: 0.00001%

as we can see, the amount of Chi gained is highly concentrated around the middle. in about 93% of the cases, you will be getting 2-4 Chi.

This is a significant improvement to how it is handled on live!

ps: as a side note, this numbers also reflect the ingame tests Chimerian did, so I we can safely assume it's correct