In Nightmare, I think ilvls are based on mlvls...you can check out the mlvls at d2data.net. Of course, you can get ilvl 85 items from "The Pit" in A1 Hell, if you can handle the monsters there, which is the same ilvl as you'd get from A5 hell world keep. If all you're after is +2 tree rares, though, it doesn't matter where you get them from at your clvl, so long as it isn't A1 norm.
If you really want to craft quality amulets, and have a chance of getting an uber +4 ammy (+2 class, +2 tree...just hope they're both for the same class if you're ever lucky enough to roll both prefix groups together), you'll want to reach Clvl 93+ and craft gambled ammys. (at clvl **, you'd have a 10% chance to get the ilvl required, at 92 you'd have 90% chance, in-between each level makes a big difference)
Of course, you could already get a +1 class, +2 tree ammy, which would also be highly valued, if it has a couple other decent mods.
Edit: noticed you said "nightmare", corrected for relevance