I feel like it is best to use it for the solar chakra because it is the powerhouse of our souls but the description doesn't specify any connection with the sun or the solar chakra so you can experiment with it.
I personally prefer to focus on the solar chakra. I vizualize it getting brighter and brighter and once I am done I then circulate the energy I gathered in my entire soul.
It feels very empowering!
In the updates Hp Hoodedcobra intends to do on the Jos, I really hope he is going to include some more information about the Necronomicon and how to properly utilize it!!!
