Asus Maximus IX Hero Overvolting CPU

Hello all, I recently bought an Asus Maximus IX Hero to use with my i7-6700K. I started with a very small overclock to 4.2GHz and put the CPU voltage in manual mode at 1.28V. However, when I use i7z to look at what my CPU voltage is, it goes up to over 1.4V under load. This seems high to me and how do I properly set CPU voltages on this board? Thank you.

Did you manually set 1.28v (seems low) as well as adjust Load Line Calibration?
What is the max stable clock at stock voltage?