If I read the picture correctly, it does not seem like there is any adjustment available at either end of the return cable, the only adjustment - if there is one - being on the cable itself near the throttle grip.

The same seems to hold true for the opening cable, but here there is an adjuster guaranteed.

From what I have learned, if the return cable can be adjusted you set it to zero slack. Note: Not zero play of the throttle grip, but no slack of the cable. Make sure the throttle has play, and that the throttle returns snappily. Then set the play where you want it by using the adjuster on the opening cable.