From RedTeamMiner documentation:

To be able to lower voltage properly, you need to modify the powerplay table
   with e.g. MorePowerTool. Install GPU-Z and MorePowerTool. Save the bios using
   GPU-Z. Open MorePowerTool, select your Big Navi gpu in the dropdown list. If
   necessary, load the bios to get a baseline configuration. Lower the "Power
   and Voltage" -> "Minimum Voltage GFX (mV)" limit to the voltage you with to
   run. Lower than 625mV is rarely stable, although we've been able to run at 612mV.

Then you Save, Restart Windows and you will see the new minimal core mV to the number you choosed.