Move the slider so far to the right that only a few white spots remain in the
document. Use the “Color Sampler tool” and set down a point there. Move
the slider to the left until only a few black spots remain, and set a second
point down there.
One could also find a neutral gray in the image by using a “Threshold
adjustment layer.” Add a new blank layer between the original image and
the threshold adjustment layer, and fill this layer with 50% gray. Go to Edit
→ Fill or press Shift + F5, then select “50% Gray” under “Contents” and
click “OK.”
Here is the threshold adjustment layer at work.