Well if the only success was with off hand did you notice any changes in the numbers? For example did the average fail on MH give 5% increase at 15 durability and the fail with OH give 9% increase at 14 durability? This is what makes it easier to determine which hand.
I just did it twice with main hand. Failed first, succeeded second. 14 Armorsmith with guild assist. First attempt hit 100% just as I hit 0 Durability. Second attempt got 100% with only 9 durability left over. I might try in the future with off hand, but if this is a mainhand recipe than it is definitely not 11. Most likely 13-14.
I just had 1 success out of 5 with this as a level 14 Armorer with common synthesis support. I do not believe this is level 11. I tried both tools and the only success was with Off Hand.