Purchase Order Blanket - Automatic Calculation on Amt Agreed field

Dear All,

in purchase order form, if we change Type to "Blanket Purchase Agreement" then "Amt Agreed" field will appear. the field is not mandatory so we can leave it blank.

business process in my company require that field is mandatory and the value are summary from field "Price" in Purchase Order - lines.

is there any setup to make the field to be like i needed?

or if it had to be done through personalization, how to sum "price" field on PO lines before save the transaction?

Regards,

Suryadi