I tried it at 1 and 2% and really love it a 2%. I used 6 drops for 15ml at 2%. 3 drops for 15ml at 1%. I looked at my calculator and it states 1.2 drops for 3ml which is 2%.
The drops per ml really depend a lot on the dropper top and drop size though. Unless you measure how many drops equals a ml from that particular bottle, it is hard to judge by drops IMO.
I have a few tiny .05 ml syringes just for measuring out small amounts many people do go by drops and get good results though.