Calculate card charges to the Grand total

Hi i have created a custom field as card_charges to sales order form. I need to add the card charges value to the grand total in the form. Can anyone please help me how to do this.

Use cur_frm.set_value()
cur_frm.set_value(“grand_total”,cur_frm.doc.grand_total + cur_frm.doc. card_charges )

Thanks, Sukrut
New Indictranstech