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