There is no set "Safe Min Voltage". The higher the overclock, the more voltage you'll need. To confirm that your settings work you need to stress test the CPU with something like Prime95, IntelBurnTest or OCCT. If it's stable, you can either up the multiplier or decrease the voltage a notch and then test again. You'll know when you've got to far in either direction because it will fair during the stress test. If that happens then go back to your last known stable settings and retest.