The calculator allows you to cast any spell you've learned from any job on anyone on the screen, provided they meet the guidelines you choose. For example, if you used Math Skill and slected 'Level', '3', and 'Holy', all people on screen with a level multiple of three will be hit with the holy spell (including your players if applicable). The greatness of it is that it does not take charge time or mp to use these spells, so if you play your calculation right you can do some real damage on the enemy from a far distance, and, like quick reply, get immediate results.