Can there be more than 1 unit with this passive?
Is it a learned passive?
Does the unit exist in start of map or do you "pick" a hero?
Is it even a hero?
The basics are:
1. Have an initialize trigger, if it is a learned ability or you have a hero picking system, add the hero to a unit-group (or unit if only 1)
2. Have a timed trigger where you have your interval (every 1 seconds? 5 seconds?), for-each unit in group, have an "if" checking the hp if current hp is greater than 0.2 * max-hp, if so set current hp to (Unit - Property (Life)) + (0.02 * (Unit - Max hp)), otherwise set it to the 0.06 value...
Have you tried doing that?