the Jester
Legend
1 round of cone of cold against three targets is 36 * 3 - 108. Two rounds of [x] against two targets equals 17 * 2 *2 = 68. 108 + 68 = 176 / 3 = 58. I'm then supposed to add the damage from her freezing aura (5), any legendary actions (one deals 20), and lair actions (one deals 5). So that's 58 + 30 = 88.
Assuming your numbers are otherwise correct, I think the lair action and the aura damage might be higher in actual play. If the pcs have two melee types, the aura is going to deal 10 per round, not 5; and if she can keep moving and forcing the melee guys to take spike growth damage each round, even just 5 points, you've gone from 5/round to 10/round, as she's making the two melee guys move 5' each.
Anyhow, it's a minor technicality in this case, and I don't know your party's makeup or weapon choices. But you might consider re-evaluating these two elements if the party has multiple pcs focused on close melee combat.