  I don't know if the tooltip's description completely escapes you, but it's based on the DRUID'S base mana pool, meaning it doesn't matter how much spirit or mana regen you have, the amount of mana you will regenerate from Innervate will be based on the DRUID'S base mana pool.  This also means that it doesn't matter who you cast the Innervate on, the amount of mana regenerated will be the same regardless of class. The only way the amount will change is if the druid who cast the Innervate on you wasn't 00, because of course, the Druid's base mana would be lower.  