While you are making sure that you are keeping your foot on the brake, put the car in reverse. While it is in reverse (and you are keeping your foot on the brake), use the mirror adjustment buttons to adjust each mirror to the position that you want it to go to when you are backing up. After you do that, those positions will be memorized, and the mirrors will move to those positions when you are in reverse.
Note that, if you change the normal mirror positions, you will need to re-set the reverse mirror positions.
(Also note that you posted in the NX forum, and it appears that you have an IS.)