1 cartridge a day at 16mg is way under average. I wouldn't worry about that being too much.
I'm not sure what you've been told about the nicotine content, so here's roughly how it goes:
1 cigarette has roughly 10mg of nicotine in it, and you absorb around 1mg of that. That is what the mg rating is for on the cigarette boxes; not the total nicotine content but the average amount absorbed. So you can say there's about a 10% absorption rate.
Now you see that 16mg is 16 cigarettes worth per millilitre
after absorption but you still have an absorption rate to go for ecigs as well. There is no set level for this with ecigs but it's generally assumed to be around 10% as well. Therefore, that one cartridge if it's 1ml in size is roughly equal to one and a half cigarettes of absorbed nicotine; 10% of 16mg = 1.6mg and 1 cig after absorption was 1mg.
I need 8 - 4mg but up to 4ml of it in one day. One standard cartridge is maybe around 0.5 - 1ml so you aren't getting much nicotine really. The average amount vaped per day is around 2 - 3ml.
With all that said and done, what really matters is how much you feel like you need.
* (proof for average amount vaped)
Hope that helps
