A good rule of thumb I found with my setup is the power to memory clock ratio.  1 to 10.  If power limit = 75 then 750 oc memory clock.  I just started mining today but I'm not entirely sure what oc settings to use for mining ZERO.  But these have been my settings for mining equihash algos.  If anyone can chime in please add your settings for mining ZERO.  Thanks =).