How to check your paintings values in Photoshop?
A quick tip I learned on Tiffanie Mang workshop is how to easily check your paintings values in Photoshop.
- Go to View > Proof Setup > Custom;
- Under Proof Conditions > Device to Simulate: select Dot Gain 20%;
- Click Ok.

Now if you hit CMD + Y (CTRL + Y on Windows) it triggers the gray scale value version of your painting. Use the same shortcut again to go back to the normal version of your painting.
For me, this is way easier than creating a new layer, filling it with black color and applying a screen blender mode to it, but your mileage may vary. I also heard that using Dot Gain 20% is more accurate, so, win win. :)
Cheers!