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First you need more hand control, and there is no formula. Second, try to make hatch follows the shape.
Also you can use more art materials on same image.
Since this one it is a side view there is not to much room for
makeing volumen with light
try makening a more isometric aproach to your tank-robot.
I think your shading tecnique is pretty good but change the anlge so that we can see the volumen =) i hope this helped a bit.
I really like the design =)