I also feel comfortable with basic chemistry so I ordered the 100 mg/ml for my first attempt.
However, being comfortable with it and even having the wonderful calculator and being careful isn't enough for me. Once I had my formula and mixed my first batch, I TESTED the nic level with the nic kit I bought for about $9. And I was right on. I feel better now KNOWING that, and will never wonder again if the nic I bought really was 100 mg/ml and the mixing I did was correct.
Once I had my basic mixture down, I make a batch of it and then just add my flavorings into the smaller bottles.
So, basically ordered 1000 ml (liter) of 100 mg/ml nicotine, transferred it to the 50 ml european stopper BROWN bottles (which I don't bother to label because in my house any brown bottle will ALWAYS be 100 mg/ml nic). Then I take the recipe I tested and make 50 ml of base in a BLUE bottle, and label that with my base recipe and date. Now I know exactly what is in what colored bottle. Since I am not making large amounts of finished product yet, this allows me to test batch flavorings into smaller bottles without having to handle the 100 mg/ml any more often than necessary (and it is in 50 ml bottles to won't have a terrible spill of a whole liter!). I only handle it when I need to make another blue bottle of base.
My base has been tested, the 100mg/ml nicotine is secured in smaller quantities and I feel confident. I also did take into consideration the flavorings I would be adding which will also bring the nic level down, so my base is a bit above what I
vape. Since I am not mixing it for sale I am okay with being ~3% on the nic level when I account for less or more flavorings.
This may sound like a lot of extra work, but my safety and confidence are priceless and I can whip up a flavor in minutes.