The ilvl on the charm is most important for cubing. It determines what affixes you can get on the charm. If you cube a high ilvl charm you can get any of the mods. If you want something like a skiller GC I prefer to use a middle ilvl range to limit the affixes possibilities.
This should help you out. Make sure you read through the whole thread though, as Thrugg points out some mistakes in the ilvls.
If for some reason you don't want to read through the thread, anything over ilvl 50 for gc/scs/lcs will pretty much get you what you want. If you don't know ilvl 50 starts in Act 3 nm. Hope this helps :thumbsup: .