The key is to decide exactly how dark and how light the region is to be. Start with the dark, and make a small area exactly as dark as is needed, maybe even black. Then partway between the darkest and the lightest area, hatch half as dark as the darkest area a small part of the medium value. Hatch the medium value all the way to the dark value. Then gradually build up the area nearest the dark value with more hatching. As you go, you should see the medium hatch become darker. Strive to make the darkest part near the black edge or core. That's half the shading.
The other half goes from medium to light. Partway between the medium area and the light, apply a small area of hatching that is half as dense as the medium hatch. Build that into the medium hatch the closer you get to the medium zone. Then apply light hatching to finish the transition from light to medium to dark hatching.
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