retro in PCR

Dear Experts,

I need clarification on below.

For Example, payroll have already been run for in period 10 and for period.

and following WT 7770 have already been created..

My other PCR is calculating amounting amount from previous period (Only applicable for current month).. Normal case i am using AMt=L 7770. which is giving me correct value.. But in case of multiple period in payroll run. it is not giving me correct value.. because a line is added in RT without this wage type.. 

1) Can i write the PCR which runs on Retro period and copy the same value which have already been calculated in that month of live payroll.  

OR Modify my existing PCR line (Amt=L 7770) that should calculates previous month value in which for periond and in period are the same ?

Regards,

Muhammad Umer