Of course, I'm always happy to help!
In general I think that rule is good to follow when doings that are from imagination like this. I think there are rare cases where this isn't true, but I think it is when it comes to bounce light. I eye dropped values from shapes and the bounce light was darker than any value on the lit side. One comes close, but it's still slightly darker.
You bring up a good point with your sphere though. I think for 10 I'd say it's good to go lighter on the bounce light, but for 8 and 3 it's better to make the core shadow darker instead. I should've mentioned that I had to use a darker value for them.