While you're leveling the calculator and getting the math skills, make sure to put on gained JP up and accumulate, and some gear that adds to speed, Calculators are the slowest class, and so you wan't to of course get more turns in so you can get more JP. Yell them with Ramza to get them going even faster. If you have move-JP up on, then can get even more jp just by moving around. If your calculator is a regular character (not a story character), you can send him/her on propositions to get easy fast job points. But, since regular characters are pointless, you'll want to give a different character Math skill (preferably someone fast with good MA, like Beowulf or even better, Reis). Also note that distribute and damage-split are very useful in some cases.

For using a calculators skills, put them on auto battle, and they'll choose in seconds what the AI sees as the best possible move.