Check out this Gear Enhancement Probability Guide for Dragon Nest by Puru:

Note: You can go down in Levels on Epic Level 5s and Rares can go down at 8

Item to +1
100% Success Chance

Item to +2
100% Success Chance

Item to +3
100% Success Chance

Item to +4
80% Success Chance

Item to +5
60% Success Chance

Item to +6
50% Success Chance

Item to +7
45% Success Chance / 25% Break Chance

Item to +8
40% Success Chance / 25% Break Chance

Item to +9
35% Success Chance / 25% Break Chance

Item to +10
30% Success Chance / 25% Break Chance

Item to +11
25% Success Chance / 25% Break Chance

Item to +12
20% Success Chance / 25% Break Chance
Failing can result in losing 1-2 levels.

Item to +13
15% Success Chance / 25% Break Chance
Failing can result in losing 1-2 levels.

Item to +14
5% Success Chance / 25% Break Chance
Failing can result in losing 1-3 levels.

Item to +15
1% Success Chance / 25% Break Chance
Failing can result in losing 1-3 levels.

Source

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