Headachy, clearly "buzzed" -- too high.
Vaping like a smoke stack, still jonesing for an analog -- too low.
Generally content, liking the vape but not dying for something -- just right.
I couldn't stop smoking on 16 -- just not enough nicotine for me, particularly at trigger points (first up in the morning, first time in the car, right after dinner).
At 24 I could stop, used a last pack up that week, couple a day, but didn't have a strong urge (just cheap -- I bought 'em, I'll smoke 'em)
Now my go-to preferred blend is my own tob4 at 20 mg strength. It just plain tastes good to me. I love flavors, trying new things, but anywhere from 12 - 30 mg still gets me nicotine; I use 12 generally on a pass-through at work, maybe one puff every 5 minutes, with 20 or 24 in a battery. 30 is equally nice once in a while, but I wouldn't normally chain-vape it; I just don't need it. I'm not edgy, zero cravings. I will generally drop to a lower nic late at night, but it doesn't seem to matter that much, last night I had a couple puffs of Hunters 24 just before I went to sleep, because I like the taste.