Hi Umer,
Is not clear me about your requirement. Since AMT=L 7770 is reading only the last month payroll result or you want to bring last 9 months cumulative value in WT7770 then check AMT=C opeartion where WT7770 PC30 should be T for table T524C1.
If retro happen and you want to bring difference in the separate wage type for 7770 the you can modify X042,X043 & X044.
Check once and tell me.
Regards,
Sankarsan