how to print Signature in smartform in a dynamic place ?

Hi experts,

                i have a requirement to print signature in a smartform (purchase order print) ,the signature has to printed in dynamic place.

                i have  to Print signature in in authorized signatory place, we cannot place static window with scanned signature in page ,

                as we cannot predict where the page will end.

                how can this be attained, i have attached a print preview , where signature has to be attached  ?

               please Help.