That depends greatly on how good your chip is. I got mine on 1.2V but there are people on here that have hard a hard time getting that on 1.3V
The best way is to slowly work your way up and see what is stable for your chip.
What should I start with?
Start with stock voltage and then bump the multiplier up until it is no longer stable. Then voltage followed by multiplier until you get where you want it.
Set multiplier to desired frequency put 1.2Vcore in and see what that gets you and adjust.
remember stay under 1.4Vcore and 80C prime95 for 24/7 OC