Whats the best way to undervolt my cards?

I have 2 x MSI ARMOR 4GB RX570 and 4 x Aorus 4GB RX570 split across 2 PSU. From the wall i'm pulling 1010kw so very keen to drop this as low as possible. Getting 161mh/s from this rig and at  up 24Kw a day @ .30c per KW for power it's expensive, even with solar.