I don't have a puratek, but hopefully this will help.
T off for the drinking water, anywhere after your RO membrane, and before the DI unit. Where ever is more convenient for you. The water after the DI is solely for your aquarium.
Here is a link that shows a pic. Its the pic after it shows you how to connect the drain. (scroll down)
http://www.melevsreef.com/rodi_install.html