After some mission i tested,
Seems like 15-20 is the default damage, and below that is the real damage multiplier.
So for me, set to 15-20 at battle start, set for 13 for damage multiplier, then change back to 15-20 when battle finish.
If you forget to change back, your damage will be 1 again at next battle. Set to 15 then retry the battle.