Well..
Two ways you can do this. The strange but quick/ slightly dangerous way is;
Put the voltage the 1.35 and slowly lower it by like .02 until the clock becomes unstable (Crashes PC/ bluescreen) then increase it by .02.
Or, stock everything, increase the clock a bit, when it becomes unstable increase voltage by like .02-.03 and then increase clock more until unstable, and repeat until you get what you want.
It isn't smart to go above 1.35v for a 24/7 clock