Total Ink Calculation

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From the symbols at the bottom of the Layers panel, select Adjustment Layer and from its drop-down menu choose Threshold. You will be presented with a histogram, and below it, a slider. Move the slider all the way to the left. The image will go completely white. Inch the slider to the right until the first black patch appears.

Choose the Color Sampler tool. Click the black mark. The Color Sampler will mark the site. You no longer need the Threshold layer, so drag it to the Trash. The point you marked will remain and be displayed in the image You have now successfully located darkest element in the image.

Now, look in the Info Panel. Click and hold on the eyedropper labeled #1. From the dropdown menu choose Total Ink, which is the value of the point in the image you had marked.

{EDIT] click for larger size image

Something old to visualise the total ink exceeding a certain value:

Re: maximum total ink