Hey Penny, one of my rigs is running the 570 Nitro+'s as well, here's what I learned.
1200 core is the sweet spot on these, looks like you've found it!
Use GPU-Z to check ASIC quality of each card, it is very tightly correlated to how well they'll undervolt. The cards that I have that are ~79% ASIC quality run 836mV at 1200, ~75% runs ~880mV, etc, but I also got a crappy 67% one that wont undervolt at 1200 at all lol, its running 1150 at 930mV
1180 is the best spot here.
Msi 580 8G @ 30.1, Msi 570 4G @ 28.7, and Sapphire 580 8G @ 29.3~29.4
how can I reduce the voltage to 900 in bios while flash!!! (newbie)