As B.t.V. said, Calculators have very low speed. (It's the lowest of any character class, I believe.) Give the Calculator a Thief Hat if you have some, and have Ramza yell, yell, and yell some more at him/her. Always keep the Calculator under the effects of a Haste spell. And put Gained Jp Up on him/her as well. That will speed up the mastery of the Math Skills set. As soon as you've mastered Math Skills, stay away from the Calculator job, just use Math as a secondary skill.
Try this leveling technique:
You'll need a Calculator who has learned Height and at least one number variable (I like Prime Number, myself.), and give him Time Magic as a secondary skill. Take the Calculator into battle and kill every enemy but one. Then position the enemy and your other characters on prime numbered heights (by using Don't Move, Stop, Sleep, etc.) or whichever number variable you've learned. Once the others are all stuck on that height, you can put them all to sleep by calculating Height X Prime Number X Sleep. Everyone except your Calculator will be asleep for a few rounds. Take the turns your Calc gets to cast Haste on yourself over and over and over, gaining Jp with each casting. You'll master Math Skills after a battle or two, and then you can stay away from the straight-up Calculator class.