You cannot directly map GPU Boost 3.0 overvoltage % to millivolts. % maps to different offset in millivolts depending on the card and even on the same system and card 100% overvoltage value may be scaled down to lower offset in mv depending on thermal conditions.
100% value is the difference between the maximum safe voltage (which is defined by BIOS) and GPU operating voltage (which depends on silicon and which is different for each card and defined by GPU leakage level (ASIC quality)). And as I said above maximum voltage can be also dynamically scaled down depending on thermal conditions.
For example on my 1070 card operating voltage is about 1040mV and with GPU Boost 3.0 overvoltage set to 100% it can reach up to 1090mV.
The best way to know your operating and maximum voltages would be setting power and thermal limits to maximum, setting fan speed to maximum to exclude 100% overvoltage downscaling, then running some not too heavy 3D application (which should NOT be power limited). This will cause GPU Boost 3.0 dynamic clock control to be limited by operating voltage (Voltage limit graph in monitoring window should report "1" in this case). Then you can set overvoltage to 100% and see how much voltage will be added.
The following video demonstrates that:
https://youtu.be/SAHWUt3zbGM
