Q u o t e:
According to Kavan on EJ, this theory of aiming to maintain your mana above some percentage is wrong anyways. If there was a magical percentage, it would be 40% because the idea is to gradually ramp up your cycle's mps as you get closer and closer to using evocation. What matters is the time at which your evocation comes off cooldown, not some magical percentage point at which mastery becomes more or less effective.
I think that's more accurate. Generally speaking, the approach is something along the lines of...
Evocation and mana gem are available at the start so you burn some mana quickly, use gem and dps cooldowns, burn more down to 40%, evoc back to 100%, play conservatively until mana cooldowns are available, burn more, then come back up and play conservatively, then nearing the end of the fight you try to time your burn down once again to match the end of the fight.