When changing your Job, V&C Dungeon Finder will register not register and update the first job change. It will keep your initial start. Afterwards, when you change your job again, the item level you are currently on will be detected. Behavior seems to be like n-1 with the rule it can't be less than 0 or negative.

Example:
0-1=0 (610 when white mage)
1-1=0 (610 when sage)
2-1=1 (392 when white mage)
3-1=2 (610 when Dark Knight
etc.

sage = 392
white mage =610
Dark Night =605