The maximum level of the support job is fixed at half of the main job's current level. If you create a character with a level 10 Warrior as the main job and a level 30 White Mage as the support job, the White Mage level will be reduced to 5 when set as a support job. Conversely, if White Mage is set as the main job and Warrior is the support job, the Warrior level will remain at 10 even if set as a support job, because the White Mage can utilize a support job up to level 15. Keep the above in mind when setting a support job so that the two job levels are balanced to your advantage.