Removing Midtone Color Casts By Finding Neutral Gray In a Photo With Photoshop

I should remove color cast more often but I usually forget how to do it. Photoshop Essentials has a very nice tutorial with lots of pictures and I'm hoping if I take some notes it will finally stick. The images on the left are the originals and the right was corrected.

 I took this photo of Lake Itasca and the Mississippi River headwaters on a very overcast day. You may notice the extra detail in the water that is now visible in the right image. The black and white are easy to locate but guessing where neutral gray is located is significantly more difficult.

The vintage black and white photo has a few extra steps since the color was also rather greenish. My notes will start here:

To make the photo black and white:
Image > Adjustments > Channel Mixer > Monochrome

Image > Adjustments > Levels
Use the eyedroppers to pick the black and white tones.

Neutral Grey:
Add a new layer.
Edit > Fill > 50% Gray
Blend Mode > Difference

New Adjustment Layer > Threshold
Drag the white slider all the way to the left (the image will now be white)
Slowly drag the slider right until you get a click-able size patch of black
Click on the black area with the Color Sampler Tool. Zooming in closer may be helpful. Remember where you clicked since the target mark can be difficult to see on some photos.

Delete the 50% Gray and Threshold layers.

New Adjustment Layer > Levels or Curves
Select the Gray eyedropper.
Click the target marker on the photograph. The mid-tone color cast is now neutralized.

 You can clear the marker but it's not necessary.