How much nicotine to use depends a lot on your equipment, as well as. I tend to like/need a bit more nicotine than a lot of people, and I use some really different setups at different times. I mix different levels for different setups. With even a pretty decent MTL setup like the Nautilus 2 with the 1.8 Claptons at around 20W I prefer 12-18mg, but for DTL dripping, around 0.4-0.5 ohms at higher (but still pretty low- like 35-50W, depending,) I usually mix at 6mg. I also mix different recipes for different strengths, because subtle recipes don’t stand up very well to 18mg… Don’t take the ohms, watts, etc. I mentioned as too significant though, because there are a lot of other factors involved. You just have to experiment till you find the right level for your needs and equipment.
One thing I think is that if you’re still smoking, and want to stop completely by vaping, it’s better to err a bit on the high side with nicotine (one of the nice things about DIY is that you can experiment with this really inexpensively, and be much more sure of the nic level you’re using than you could with commercial juice.) I’d push it up until you get a bit light-headed, maybe even just slightly queasy. You can always pull it back later, but… there are enough things you’ll miss about smoking at first that you don’t want nicotine to be one of them.
I started vaping with 24-32mg juice. The devices back then were much less efficient, and 32mg especially would be very high for any modern setup. But it was still pretty high even on the eGos of the time, and I think that really helped me. I was a pretty hardcore smoker though.