There are actually several factors that can determine the nic strength you are happy with.
Different delivery devices can make the nic strength need to be adjusted. Usually, I've seen this mentioned when folks use the more advanced RBA's. Zen mentioned it on a video I was watching the other day when he was demonstrating his atty over at the Purevape site. He mentioned you will probably want to cut your nic down.
Vapers often vape a lot more than they smoked. I think just because it tastes so good, compared to cigs. So, heavy vapers may need to cut down on nic or else get nic sick.
If your goal is to cut down on nic, that can be done too. I mix 2 levels of the same flavor together to gradually reduce the level of nic. You get so used to the newer "flavor" and throat hit that way so you don't even miss the higher nic. In fact if you go back and try a higher one at some point, you may just choke and wonder how you ever liked it. I personally started at 24 mg and after 6 months of transitioning down, I'm using mostly 0 mg, now.