Also when one finds a sweetspot for undervolt mV value, is applying this mV value to all GPU speedssets other than the first set (300 Mhz-750 mV) best practice?
The difference is so small (750 -> 831) that you gain nothing by making voltage steps. Also, this is mining, so you will be always in the last state.
some algos switch between less intensive iterations though so I belive the last and second last states should be modified just to be on the safe side.