1. Open image in Photoshop.
2. Go to Image / Mode / Grayscale. You will get a message asking “Discard color information?” Click OK.
3. Go to Image / Adjustments / Curves. Double click on the eye dropper for the black point. Change C, M, and Y to 0%. K should be set to 93%. (This will be the darkest point on your halftone.) Click OK .
4. Next, double click the white point eye dropper and change C, M, Y to 0% and K to 3%. This will be the whitest point on your halftone. Click OK.
5. Now, click the Auto button. The highlight will be automatically adjusted to 3% and the shadow 93%.
After selecting OK, choose yes when asked to save as defaults. The next time you convert to grayscale, just open curves dialog box, and select Auto, the 93% and 3% will now be the defaults.