Hi! I don't think I can help out too much because I think a huge factor is RNG and varies person-to-person, but
for me personally I've gotten about 30 [this is a guess, I've sold too many to get an accurate and exact number] adult not-cats from Nestor. About 2 of those not-cats were beans (I think, because Cake and Acre are Nestor kittens). Based on these numbers I think I'd have a 1/16 chance. Based on these numbers...me getting a kitten is a 6.25%. However! It's also important to note that I got Acre before the 1 week guarantee cat. So I'll take out Acre for this. So...then...my probability would be...3.2%(ish).
I think it depends on person to person, but your question honestly fascinates me! A lot of data has to be done...but I do think someone out here on the forums is collecting bean data (I could be wrong)!