How did you Sending Item details to the Smart form.
Definitely You create a table structure , In this table you are appending the lines then you to assigning to the smart form.
In the same way in driver program create one Variable like Doc_total.
then in Driver program calculate the value then Sending this value to the smart form. Like below
In smart form Form interface you have to create the Same variable like
doc_toal EKPO-NETWR
Then in the Driver program you have to assign the Doc_total to the
CALL FUNCTION fm_name1 "'/1BCDWB/SF00000005'
EXPORTING
Doc_total = Doc_total "Driver program variable.
IMPORTING
job_output_info = w_return
TABLES
it_item = it_item[]
IF sy-subrc <> 0.
* Implement suitable error handling here
the the doc_total will be come into the Smart form. Hope you understood.