Mp refresh at 70-80 MP is a good rule for the learning curve.

With experience you can adjust to get more out of your mana. Mp is at 90% and you're going to be spamming Cure II/Benefict II or Holy/Gravity? Pop SoS/LA so you can break even at the end of it and SoS/LA will be ready again sooner. On the otherhand if it's the end of a pull and you can be at ~full mp by the time you start the next pull, save it.