I am on an older plan, so cannot add the add-on. However for over two weeks and across a billing period no matter how hard I hammered the downloads my usage for the day was no more than 100MB. I asked here (didn't ask via official support channels), got no reply and so just went with it.
my usage was out for 3 weeks , most days i only used 500MB a day even though i knew i had downloaded a 20GB file for a beta on one day and my free youtube never moved off 2.53 GB. it happens most months but usually for only a couple of days