I am sure I saw this or something like it in a book somewhere so I am not claiming it as an original idea, but I think it is a good idea and I want to share it with you. It is what I will call a ValueMate. A tool used to help you determine the values in your desired painting subject.
How to make a ValueMate:
Go to your local hardware or home paint store. Once there, look for a color swatch with "black" as the darkest color on it. These usually come in five, six or seven gradations from the dark to a light value. I prefer to use a six graded swatch just to keep things simple. These swatches (or at least the ones I have found) don't have pure white in them. That is something you will have to keep in mind when you are using them. Anything that registers off the value swatch is simply white.
Now take the swatch and use a hole puncher to punch a hole in each of the value grades. make the hole on the side of the swatch so you can locate the parts of the subject when you are using the ValueMate
How to use a ValueMate:
Simply hold the ValueMate at arms-length up to the subject you are trying to paint or draw and try to match the elements of the image to one of the values. You do that by squinting and viewing the elements through the holes you have punched. squinting makes the colors in the subject turn to gray. Now move your ValueMate up and down until you find the value grade in the swatch that matches the value of the element in the image/scene. Make notes as you do this across the plain of the subject and your work will be much enhanced.... I hope.