You might find this tool helpful to experiment with: https://www.proko.com/values. The tool is similar to the posterize tool in photoshop and it flattens the values based on the input -- you can select 2 through 11 values. What the tool doesn't do is design interesting value shapes, which is up to you! In order to do so, you must start simply and build up to the more complex shapes. Here is a demo I did for another student that is hopefully helpful. I start very simply by finding the separation between the light and dark values. I can add values and carefully put together the value shapes to make the image. You definitely have the right idea. I think if you simplify a bit more in the beginning stages it will help you keep your values organized.