This is a quick set of instructions on how to set the mid-tone points of an image.
Open the image
Next create a new blank layer
Next go to – edit fill – fill with 50% gray
In the layers palette change the blend mode to “DIFFERENCE” (DO NOT BE CONCERNED THE IMAGE WILL LOOK UGLY)
Create a new adjustment layer – threshold
Move the slider to the left – the image will now be totally white
Drag the slider to the right till the first black spots appear – this is the mid-tone area of your image
Mark it with color sampler and then trash the threshold layer
Here you have your image with the mid-tone point selected