Cash Discount to be calculated on amount excluding Miscellaneous charges

Hi all,

I want certain modification to be done. Pls help.

I want Cash Disc field on Sales Totals form to show Cash Discount that is excluding the miscellaneous charges. Cash Discount should only be applicable and calculated for the SubTotal Amount. It should not be adding any miscellaneous charges. How do get it done. Any help is deeply appreciated

Thanks,