I'm seeing needing 45ml of zero nic......Ejuicemeup under Tools.
http://ejuice.breaktru.com/
You'll be close though, at 8mg/ml.
- Of course, wait until someone double checks my figures....
In simple terms- Another equal amount (15ml) in zero nic divides the nic to 12mg/ml and then doubles the amount to 30ml. Then you'd need to match that amount (30ml) to cut it in half again to 6mg/ml (60ml).