Values are relative. Don't try to measure values when you're trying to sculpt form. After you rough in your planes, just control and refine gradients until it looks correct.
The more a plane faces the light, the less value change you need to "bend" it.
Probably gonna get some animal skulls to still life it up.
Ugh.. could not get that cheek/forehead to look right for the life of me. I'm so sorry.