The Evoker in Volo's Guide to Monsters (p. 214) is listed as CR 9. However, I just can't figure out how to calculate its CR to get that high a result.
Spell Attack: +7 & Save DC: 15 with 75 DPR average = Offensive CR: 11
(Cone of Cold: 1 at 6th Level, 2 at 5th Level)
So the best overall CR I can get is CR 6 without Stoneskin, and CR 7 with Stoneskin.
I'm not sure how to boost it to a CR 9 in my studies. I was planning on making a slightly stronger Evoker, but trying to get the baseline down.