Caporegime
Leaving it at default 950mv should be fine in my experience. Just focus on increaseing P7 core voltage and dial down until you hit instability.Yes. You need to raise the floor voltage (the power indicator under the HBM) to 1000mv to overclock HBM.
Of all the Vega samples i have had (6) some required 1225mv to run 1100Mhz and some would do it on as little as 1125mv pn P7 core.