Eligibility profile based on a task is completed in a Checklist/journey or not

We need to make a Eligibility profile for a performance document based on the condition that is particular task is complete in the journey or marked not applicable
If completed - Eligible

If marked not applicable - Not eligible

I think we can use a Fast Formula but need help with which database items / inputs / variables can be used in Fast Formula to retrieve the status of a checklist task?