This test was done on a hero with 0 armor reflecting back to a hero with 0 armor, so armor is not a factor in these tests at all.

First, just the passive alone.

The amount of damage returned doesn't match the amount of damage taken by the returning player. There are slight variations between expected values, and occasionally, it feels like the flat 20 damage is not applied at all (see the 4th and 5th instances doing way too little damage)

Now to the active. From the wording of the description, it should cause the passive to increase to 20 flat + 100% of attack damage taken. For other sources of damage, it should be 80%.

As you can see here, there are some deviations as well. Like before, the numbers are slightly off, and sometimes the flat +20 is not applied at all.