sub-routines in scripts

Hi Everybody,

i have copied the standard script F140_DOCU_EXC_01 into zf140_doc, in that form i need to caluculate Net amount in text element 521 in main window. for that i have written the subroutine.

PERFORM <subroutin_name> IN PROGRAM <prog_name>

USING <parms>

CHANGING <parms>

ENDFORM.

FORM <form_name> TABLES in_var type ITCSY

out_var type ITCSY.

endform.

the above code is not a complete one.i need to caluculate the net amount (&RF140-WRSHB). i am not able to calucualte the net amount, i mean from where should i get the data. and y do u we need the ITCSY stru.. could some one help me with a code.

Thanks

Srikanth