The retirement reduction is calculated automatically, but there are some things you need to know about it -
Did you have a military pension? In Arkansas, you can choose to exclude the entire military pension, or you can choose to add the military pension to other pensions and take the $6,000 exclusion.
Also, did you check that you would take the "65 Special" credit of $26 on line 7A? If you did, then you do not get the $6,000 pension exclusion.
For background, see the AR1000F instructions on page 11, line 7a, and page 12, lines 17-18.