Bex query customer exit help

My Bex query contains 5 columns :

Col1 – Prev quarter 1PQ, col2 – second prev quarter 2PQ and soon until col5 – 5’th prev quarter 5PQ

All KFs are on rows of Bex Query designer.

Report Selection screen should contain variable current quarter lets say ZCUR_Q

I am required to calculate all 5 prev quarters based on the quarter that I enter in selection screen.

There is already some code in CMOD as below. What should I Write in i_step = 2 to fullfil this functionality ?

CONSTANTS:

c_qtr_1 TYPE c VALUE '1',
c_qtr_2 TYPE c VALUE '2',
c_qtr_3 TYPE c VALUE '3',
c_qtr_4 TYPE c VALUE '4'.

WHEN 'ZCAL_QUARTER'.
IF i_step = 1.

CLEAR lw_range.
CASE sy-datum+4(2).
WHEN c_mon_jan OR c_mon_feb OR c_mon_mar.
lw_range-low = c_qtr_1.
WHEN c_mon_apr OR c_mon_may OR c_mon_jun.
lw_range-low = c_qtr_2.
WHEN c_mon_jul OR c_mon_aug OR c_mon_sep.
lw_range-low = c_qtr_3.
WHEN c_mon_oct OR c_mon_nov OR c_mon_dec.
lw_range-low = c_qtr_4.
ENDCASE.
lw_range-sign = lc_i.
lw_range-opt = lc_eq.
APPEND lw_range TO e_t_range.
ENDIF.