makes absolute sense. Your nicotine strength of the nicotine you have listed that you are using is 36. You want 12. 12 is 1/3 of 36. Meaning 1/3 of your juice is going to be nicotine if you wanted 12 nic from a base of 36. Since you're making 15ml, that's 5ml of 36 strength nicotine.
I know I've had to do a double take before on those calculators. Never had one incorrect but it's always a really good idea to double check like you did. Of course when I say I've never had one incorrect I mean the calculators additions, not user input error. (which you didn't do)
Never hurts to double check though, especially with nicotine. Too much flavor may just make a juice nasty or tasteless. Too much nicotine could be really dangerous so you did the right thing double checking.