When does Canni become more effcient then a Manastone (Hp/Mana ratio wise)

[VincentVolaju]

So Ive been messing around on a 33 Shaman with a Manastone, he only has Canni 1 right now. After watching hp/mana when using it, it looks like it takes 50hp and only gives 7mana, however on the wiki it says it is supposed to give 20mana. When using the Manastone it gives 20mana for 60hp.

Obviously at this point 60-20 is much more efficient then 50-7. Im assuming as you go up in level, your Canni starts to give more Mana, maybe? If thats the case, at what level / which canni spell, will become a more efficient hp/mana ratio, then the Manastone at 60/20?