Very good start, Mark, with the hat, in giving light and shadows, and therefore volume, tri-dimensional effect. Also the white lining at the top edge of the hat, where light usually makes a spot and gives shape to a dark surface.
1. Don't be shy to increase your light more (exagerate a bit), you can always revert after, to make it visible enough = a black suit can be painted dark grey, not black, to be able to paint shadows black and highlights amost white = see shiny boots for example on paintings of great masters.
2. Don't nécéssairily put your light source in the plan of the flat, sideways on the left of the character, but more upfront, to avoid having to paint a character half lightened on the right and half black on the right, and edges only on the right side. By bringing your light source in your right back (niot totally in your back = flash effect), the lightening on the hat will not be at the edge, but 1/3 to the right, then against darker at the edge. On this way you will also give light lightening to the left side of the hat, flat surface, therefor volume !
3. Use black for shadows, and grey, dark grey as basecolour, On this manner you can bring highlinghts and also shadows and shape your painting in volume. It gives you more freedom in painting volume....
Cheers ...