I tried to work it out mathematically too when I started. Now I go by how I feel, because you can tell when you are getting too much or too little.
It's hard to work the math because different gear will deliver nic better than others. It also depends on your vape style. A long, slow lung hit can be less nic than a long, slow mouth hit exhaled through your nose. (Even your smoking style could deliver more or less nic depending on your number of drags per cig.)
There are posts on ECF that explain vaping techniques and nicotine delivery in vaping vs. cigs. There are also great posts and research about nicotine myths that help ease excess anxiety about the effects on your health. Basically, people here seem to match how they feel with how often they want to vape. Some step down levels, and some are happy where they are. If you want to chain vape, but find your heart is racy, you get a headache and feel nauseated - you get some lower nic liquid. And keep some higher around for stress times!
Whatever you decide, it'll be what works for you. Good luck!