Digital Signature fields

How can I place a digital signature field in a pdf document, in a position that will vary each time?
 
Scenario.
A Word Document with '{signature}' string embedded where the electronic signature field is required is converted to a PDF document. Then In PDFmark code (using vb .net 2005) I need to insert the signature field. There are multiple documents and the '{signature}' string is in a different position on each document. (Length of client letter is different per document). I can programmatically insert the signature field, but have no way of either (a) arriving at the co-ordinates of the position required to add to the insert command, or (b) means of moving the signature field I have inserted from its default insertion point, to the {signature} string or a bookmark. I am using VS2005 VB and Acrobat v7 libraries.