The cable pull measurements and parts recommendations came from others on the forum. I seem to recall seeing something from Shimano about the Nexus needing 5mm of pull, but I couldn't begin to tell you where to find it. Regardless, the amount of cable a given index shifter pulls its cable is easily measured just by shifting and measuring the distance traveled. The Nexus shifter(s) should generate 5mm of travel vs 2.5mm from the index lever. I have not bothered to verify this.
pictures as promised:
DuraAce 8 speed index lever attached to a Paul Thumbie on handlebars
Cable runs from shifter to the downtube cable stop and through the bottom bracket cable guide
View of the cable passing through the BB cable guide.
exiting the BB cable guide, the cable is routed through a short piece of housing before entering the Travel Agent. The tension in the system holds this housing in place against the BB cable guide.
The cable makes its loop d'loop through the Travel Agent and passes through another short length of housing before ending at the hub.
The specific cable routing and location of the Travel Agent will vary depending on the fame. It is also very important that the cable enter the Travel Agent on the small pulley and exit from the large on its way to the hub. Otherwise, the cable pull would be reduced to 1.25mm.