You can find your own best settings as each and every card is different and it won't overclock the same.
As there is no way to increase memory voltage. Increase memory speed by increments of 25 MHz until it is unstable.
After this reduce core clock and core voltage in a similar fashion until it starts affecting the hashrate.
Use HWInfo for any memory errors and display monitor to detect if there are any artefacts visible on the screen.

My Vega 56 flash 64 air bios. Mem - 1095MHz; Core - 1246MHz; Cove(v) - 1025mV; fan - 66%