I found this artist – Jeanette Goulart – a couple of weeks ago when she offered a free 5 day class on composition.
Most YouTube art videos are people demoing products or showing their art but I really appreciate artists that actually try to teach techniques they have created or discovered so I can learn something NEW.
What I learned from this class is that there is a ratio of contrast in art that is more than just looking at value. She called it Papa, Mama, Baby. Kind of like Goldilocks. It doesn't matter which three values fall into these categories as long as the three values all appear into that ratio. So your first value could be black, (Papa - the most dominant and largest) your second value could be white (Mama - the second medium coverage) and your third values could be gray (Baby - the smallest coverage).
I found this a very simple discussion of why some compositions work and others don't.
I have painted and drawn many 7 value scales but this was the first time I though about the ratios of values.
I encourage you to check out this artist and instructor.