from my personal experiences making about 20~30 sockets in all of my time playing CO, (1.5 yrs) I've developped my own socketting trick
Certain items (earings, rings, necks, robes, boots, etc) socket more at certain levels being upped.
I've compiled a list of my own, hope it helps. (I don't take screenshots much because of the item hack thing, and I don't want to accidently incriminate myself by posting a screenshot :/)
1 socket earings... Level 67 to 82.
Successful sockets at this level (me): 3
Successful sockets at this level (about 10 other friends): 7
2 socket earings... level ??(forgot :P) to 52.
(me): 1
(others): 2
This is why you usually don't see trojans walking around with 2 socket earings unless they have an archer friend and or charecter themselves. Because the earings must be reborned to get a 2nd socket.
1 socket attack ring... level 50 to 60.
(me): 2
(others): 7
2 socket attack ring... level 70 to 90. (meaning 70>80, and 80>90)
(me): 1
(others): 5
This is one of the most common 2 socket items, because they do NOT need to be reborned.
*yawn, need to sleep now, I'll post more tommorrow*
hope this helps d^^v