You can set voltage to 1.4 and multiplier to 50, then run your stress test and if stable start lowering voltage until it becomes unstable, at that point go back up a bump and you are done. It is highly unlikely that you can lower voltage if at all.
If at 1.4 your 5ghz was unstable, you have a choice to go up in voltage and see if it gets stable, but it won't be likely at that point and above 1.4v you are risking the CPU.