Not sure about your calculation but I think I understand your meaning, so I need to add 3ml of 0 nicotine to get 6 percent?
Edit: I couldn't wait anymore

I just added 4.6ml 0nic to my 9.25ml 0.9%, if it didn't made it 0.6% I'll appreciate if somebody can tell me what's the nicotine level of my juice now
Many thanks!