I recently re-plumbed my rail to a dual feed, single return configuration. My return location uses the cold start injector port. I'm currently tuning the car, and decided to take a peek at the plugs. All looked great, with the exception of #5, which showed evidence of an overly lean mixture (whiteish base ring). This happens to be the exact location of the fuel return (i.e. location of highest pressure drop). As the injectors are relatively well matched, I'm putting two and two together here, and it looks like I need to tap a port near the center of the rail.