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

[Aaron]

Cannibalize I (LVL 24): consume 50 hp, gain 19-30 Mana (conversion ratio is 1.66 to 2.63 hp:mana)
Cannibalize II (LVL 38): consume 67 hp, gain 30-42 Mana (conversion ratio is 1.60 to 2.23 hp:mana)
Cannibalize III (LVL 54): consume 100 hp, gain 39-47 Mana (conversion ratio is 2.13 to 2.56 hp:mana)
Cannibalize IV (LVL 58): consume 148 hp, gain 81-82 Mana (conversion ratio is 1.80 to 1.83 hp:mana)