How do I get a custom button to reference the latest pdf quote in a URL?  see my URL below so far, I need a variable so the latest quote pdf related to that opportunity is passed thorugh the button URL.

My button URL looks like this at the moment:

/_ui/core/email/author/EmailAuthor?
p2_lkid={!Opportunity.OwnerFullName}
&rtype=003
&p3_lkid={!Opportunity.Id}
&doc_id=  ?????????????
&template_id=00XC0000001U41T
&p24={!User.Email}
&retURL=%2F{!Opportunity.Id}



Background or larger goal of the project:
I am creating a custom button to display on the opportunity detail that will send an email just like the email quote button on the quote detail page, but I want the opportunity to be automatically associated to the email not the quote, tha way I can pass opportunity field information in the email as well.  I want all the functionality of the "email quote" button, but on the opportunity detail page and to relate to the opportunity not the quote.  easiest/best way to accomplish this?