On this scale fields 251 and 246 must be different of background.

On this scale fields 4 and 8 must be different of background.

Scale with gray shades.

If your monitor shows these two scales as a gray color, add more red color.

If your monitor shows these two scales as a gray color, reduce red color.

If your monitor shows these two scales as a gray color, add more green color..

If your monitor shows these two scales as a gray color, reduce green color..

If your monitor shows these two scales as a gray color, add more blue color.

If your monitor shows these two scales as a gray color, reduce blue color.

Gamma - smoothing the tone of the background and the central square.

All of these color strip in PNG format can be downloaded ( Link), and more about Ad-hoc calibration read on ODG.