Wisdom to Mana Ratio
 

The Shaman wiki page suggests that each point of wisdom correlates to 0.2 mana per character level. I got an Ivandyr's Hoop today on my Shaman, at level 51 with the equation from the wiki, 0.2 * 51 = 10.2, 10.2 * 6 = 61.2, so the +6 wisdom should be returning around 61 mana, but in game I'm finding it's only netting me 23.

My character is far under 200 wisdom so I'm not running into a soft-cap issue. Am I missing a factor or is there something else I am overlooking? Any ideas appreciated, thanks!

Are you maybe replacing a 4 wisdom earring with the new one?

Nope, I've got a Black Sapphire Electrum Earring with +25 mana that it's replacing. With the Ivandyr's Hoop I lose 2 mana.