Problem with the core shadow

[nadjia]

yap, I admit, I spent a whole day figuring out how to paint the coreshadow along the terminator. It feels like a really dumb question but I cannot figure out how. Of course it comes with practice, but my problem is that I don´t know which way I should approach. I was trying with different brushes, but it was either to sharp or to soft. I just cannot figure out, if there is a little technique behind that I could practice with. Gaussian blur felt like cheating and it´s meant to be a study, and I also didn´t achieved the right effect with that.
It would be really cool if someone can help me out with that it feels like I am sitting in front of a math exercise I don´t know the formula for Probably there isn´t any deep trick behind that, but I feel stuck...

I did those with that reference:

[andrewnimble]

Hi.
A default photoshop soft brush with pen pressure opacity should do the trick

[nadjia]

Yap, that was actually easy, there was a lack of control, thank you!

[moirae]

Try drawing real things, a ping pong ball on your table rather than grey, computer generated stuff. The light generated in your reference is a glaring, unnatural spotlight. Try drawing something lit by natural light from your window, you will learn a lot more by observation.

[nadjia]

@moirae yes, I will do that. Thanks for the advice