what is the chance of getting bonus on feeding exp?

[Alan.hu]

thx 4.0, some of my hungry gear get a chance of "great success" in feeding

however, i have feed them several times, but seems the rate is less than 10%? or is there other rules of it, like using overgraded item get more chance?

if someone know about it, im begging your answer.

 

[Thinking]

it's relatively rare in my experience, probably somewhere between 1-10% chance, not sure if this depends on grade