Not trying to start a fight but I do have a question to your methodology. If you don't use a calculator are you hand writing the equation for each batch you mix? I don't see how you can just know the relative mg of the nicotine if you aren't using a calculator or hand doing the solution equations to figure out mean mg:ml ratio for strength of the nicotine in your blends. Once again I am just curious cause I wouldn't trust myself doing blind mixing.
Personally, I mix my nic base to 15mg/ml strength. Sometimes I vape it plain (no flavoring). Usually I add 5% or less total flavoring, so my ending nic is 14.25mg/ml, which is fine with me. I very rarely use up to 10% flavoring, which still leaves me at 13.5 mg/ml. That's still close enough to 15 that I really don't notice any difference. In my experience, vendor liquids are often off more than that.
I keep a spreadsheet that shows all my flavorings and standalone percents I prefer for each, plus notes I've made over time.
In another document I have hundreds of recipes I've made, some of them pretty complex.
But I never used any of those online calculators. I just never found the need.