D value is directly related to Blue Magic (hence your magic damage scales with your character level), so we’ll leave that variable out. Multiplier is related to the type of spell used against a certain mob, lets assume you hit the same mob and using the same spell, so lets leave that...
Unlike other jobs, Blue Mage must select which spells and job traits to go into battle with via the magic menu → set spells. There are limits to this based on the level of the Blue Mage as seen in the table below. Blue Magic Set Points Lvl. Points Spell Set Limit 1~10 10 6 ...
If you're a blue mage, it wouldn't hurt to have Assimilator's bazubands +1-or-better equipped to better your chances of lucking out on learning one of the Unbridled Learning spells they offer - Droning Whirlwind, Carcharian Verve, Blistering Roar, Uproot, Crashing Thunder, and Polar Roar...