I will be overclocking
Again, you can do that with the voltage offset. The only difference is a "fixed" voltage shows the actual voltage, and an offset shows how much you've added to stock voltage.
So if you wanted 1.26 volts and stock was 1.25 you would set the offset to +.01.
With a "fixed" voltage you'd just enter 1.26. Either way it's the exact same thing. It just displays/is entered differently.