Check out the patch notes for the v17 Precursor Update here: http://maplestory2.nexon.net/en/news/article/53778/precursor-update-v17
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I personally advise stacking weapons to increase your chances eg. 6/3 weapons at +12 brings the base % from 15% to 30%. Now, many may argue that this is a bad method since you want to fail as many times as possible to gain enchantment charges but in your case if you're talking most efficiently just getting to +13, then stacking weapons to increase chance is the way to go.
On a side note, I failed +11 8 times with 20% chance, but got +12 to +13 on second try with 2 extra stack weapons increasing to 25%.
Hope this info helps!