I monitor it, in that I am always aware of what nic level I am using. I started @ 24 for a few months, then went to 18. I then started DIY, and it's easy to very slowly reduce the nic. If you go slow enough, I really could not tell. Currently my main batches are mixed about 10 mg. I use a VV device, and can turn it up in the morning if I want a little more, and down at night.
For a while I kept differing levels of juices, one for morning, one for during the day, one for night, one for stress... I am happy with one flavor now, and VV makes it easy to adjust the level of nic somewhat.
If you cut down your nic too fast, you will subconsciously vape more to get more nic, I have been vaping about 3 ml/day for the last few months, some more on weekends. If I used a 30ml bottle during the day, it's hard to tell how much I used, other than averaging the bottle over X days. Now I use one 10ml or 3 ml bottle for daily carry, it's easier to see at a glance how much I am using.
On zero-nic TH? There is a thread here somewhere about extracting black pepper for TH. I love pepper, but I also find that I now (vaping for a year) inhale much less than when I smoked, or started vaping. We absorb the nic in smoking as particulate matter in our lungs. Vaping is a liquid vapor, we absorb the nic mostly with our mouth/throat/nasal/mucus membranes.
If you exhale through your nose, you will get a little bit more nic, because of more contact with an area that can absorb it.