The stock voltage should get you to or atleast close to 600Mhz. Watch your hashrate as you ramp up and your base should be  0.228 * MHz but leave room for error at higher frequencies. I usually estimate around 95% return, so at 600Mhz, you should see around 130 Gh/s. I know Sidehack likes to see 99.5% return, but that does require a little tweaking, millimeter turns of the adjustment pot, and test for days before another adjustment.