I'm not 100% sure, I think Polaris cards run with 12.5 MHz and 6.25 mv increments. Doesn't really matter I guess. When I tune my cards I just do it in 25 MHz/mv increments. So if you've got 1250 MHz core clock, then try 1225 MHz, then 1200 MHz etc. Same with voltage, if you're at 950 mv, then try 975 mv, then 1000 mv if it's still not stable etc. I think pretty much all cards are stable at 1100 MHz @ 900 mv, and most can do more. You can go either from your current unstable settings down, or you can set some safe clocks/voltages first and then try going up until the card is not stable anymore (and then you back off a bit).