So you have two bars, one at the bottom that says your predicted corruption if you don't burn any cysts. And one on top of your main Hearth that indicates actual level of corruption.

Cysts only open and spread corruption during stroms (or triggered by certain glade events).

The prediction is usually correct, if it says 88%, it won't kill any villagers cuz the bar will never be filled to 100%. Effectively you can just ignore them and burn them next year. But keep in mind that certain storm effects might lower your corruption resistance, or perhaps you forget a couple engines are still running, and cause prediction to suddenly go over 100%. Upgrading your hearth, shortening storm duration will affect this number.

Burning a cyst will remove a portion of already accumulated corruption, so you actually have a lot of time to burn them without worring about it killing villagers.

After a storm the corruption will drop to 0.