I don't understand why people say start off with a lower clock and work your way up, go for the clock you want just start at 1.25v and go up by 0.0050v at a time. So, if 1.25 does not work try 1.2550 and if that doesn't work try 1.26v then 1.2650, 1.27v and so on till you find a stable voltage.
It all depends on how good your chip is some people can do 4.5ghz on 1.25 others it takes up to 1.35v.