First off, I agree with all the others who have said that nicotine needs vary so much from person to person.
FWIW, my own experience. 12-year PAD smoker, most of which was non-filtered RYO.
Starting out vaping, I figured that non-filtered meant that I needed more nicotine, so I started at 24 mg. It didn't take too long (3 or 4 days) until I realized this was way too much.
Went down to 18 mg for awhile. Kept me off the analogs, and I felt okay, although I was getting frequent headaches and was having trouble sleeping. I was happy staying at 18 mg for the foreseeable future, until on a whim I took my blood pressure at the free machine at the local CVS. It was through the roof--higher than it's ever been!
Out of panic, I immediately cut my nicotine by 2/3--from 18 mg down to 6 mg. Started checking my blood pressure regularly, and it's better than it's ever been. I was (pleasantly) surprised at how easy it was to go from 18 mg down to 6 mg. The only thing I miss is the throat hit. I'm still getting throat hit at 6 mg, but it's not the same as 18 mg. I'm getting used to it, though.
So, as of now, I'm at 6 mg, with no plans of changing. The nice thing is that I can chain-vape in the evenings with no worries of overdoing the nic.
The takeaway: Unfortunately, this varies so much from person to person. Nothing can replace your own experience/experimentation.