You did not mention GPU, but I am using -ethi 10 on Red Devil RX470 4GB and -ethi 14 on MSI Armor RX480 4GB
I am with Sapphire RX580 Nitro+ 8GB, i use -ethi 16 but actually i dont know why.....i just dont see any difference wih or without -ethi parameter.
I tried ethi 4,8,12,16 - no difference