Multi-pass values

To build form, I work dark-to-medium-to-light, each value on its own layer, until I arrive at a final light and shadow structure. I use 70 per cent grey, then 50 per cent, then 30 or 20 per cent, sometimes with a final pass of pure white. I clip each layer to its underlying silhouette so it stays within the sil shape. This approach is nondestructive and editable.