The formulas are not known by normal people, but I can tell you my numbers after 4 years of enhancing:
A +7 gear is worth like a normal narak, it means 5%.
A +8 gear is worth like a blessed narak = 6%.
A +9 gear is like 10%.
Someone say they failed with four +10 gears, but I still don't believe in them, I never failed so I still believe in 25%.
So, even using +8 or +9 gears as naraks, you are still so far from success, considering +12 should be 0%, +11 should be 0.1% and +10 should be 1%.
But now, let's talk about the famous Urban Legends of Scarlet Blade:
- The grade of equipment does not matter.
- The location where you enhance does not matter.
- The NPC (Catherine or Caroline) does not matter.
- The amount of HP you have while enhancing does not matter.
- The pet (summoned or not) does not matter.