cru settings for cs go

hi, i have a monitor dell alienware aw2518hf and i play cs go. In CRU I would like to add a resolution of 1024x768 240h to have display scaling instead of gpu. How much hz to choose since the resolution 1920x1080 is 239.760hz. Choose the standard or reduced ?? As I set 1024x768 240hz I have such lags not when I play because then it is ok only when I look like others are playing deathcam.