How to make accurate value maps of your reference photos in Photoshop

This is a short clip of the painting tutorial I'm currently working on. Values are the single most important element in painting that is meant to look like the thing it for. All of the other elements, edges, hue, chroma, and brushwork are also important and you can't successfully ignore any of them. 

But value is the key. Trouble is, value often disguises itself as chroma, and chroma repays the favor. The edges on a high-chroma object, a lemon say, can be really hard to read because the chroma is so high. This challenges even excellent painters and I saw an example of this today. 

Painting is my life. I want to nail every painting I start, and there are plenty of pitfalls just in moving around the little piles of dirt in grease that we call paint. Perhaps it's because in art school I had to play poker in order to eat, since my scholarship didn't cover luxuries like food, but I like to put every edge in my favor that I can. 

Creating an accurate value map, especially with figure painting, is a huge boost to my production. If you ever struggle to see values properly be sure to check out this tutorial. It should be ready in a few days.