Ignoring 1 and 20, doubles will come up 18/400 or 4.5%. I'd have to argue that even given an 11 needed to hit this is more effective than a flat 5% increase in critical (19-20) because of the average damage increase. All other critical improving abilities take what would already have been a hit and just maximize the damage. If you let this hit on rolls lower than (11 in this case) the targets defense it takes a 0 damage miss and turns it into a max damage critical. The harder it is to hit the target the more beneficial this becomes because as you get closer to needing a 20 just to hit the target this approaches a +5% to the auto-hit rate (19/400 or 4.75%).