aslong as its below 1.35 its fine, but setting it lower will lower the temps slightly so thats allways good, im running my Q6600 at 2.5 on 1.1v (24 hour p95 stable), so the 8400 should be fine at 1.2v (if not lower) if you have the time to play set it at around 1.2v and then run prime 95/occt for a few hours and see if it fails, if not its good to go