I've seen it suggested @ 6-10 drops per 10ml. I tried a while back (Spring 2014) at 3.5%
It was okay, but I was hoping for more balance and lots more rum flavor. (As I like the (burp...pardon me!) drink.

) I remember it was a little bit high on the coconut in the coconut/pineapple balance for me. While I find INW Pineapple has a very nice aroma, I also find it terribly weak in the flavor department. I added a bit of my favorite stand alone pineapple flavor (from Hangsen) and that helped a bunch.
I tried mixing it again and added 2% INW mango. While I like the INW mango, it's addition did nothing for me in the mix. (Blah) With so many mixing options available, my INW Pina Colada has otherwise been collecting dust.
Just my 2 cents....
I hope you obtain far better enjoyment from it!
Ron
PS Please do not ask me about INW Grape or Whiskey. I'm a bit new here and don't wish to be booted for using foul language.

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"When you do bad things, bad things happen to you."- Farrah Fawcett