Usually when people ask this question it's because they are starting out and they don't want to become MORE addicted to nicotine than they already are. They figure that if they keep their absorbed intake the same that they will be fine. It's an ok idea but there are some really easy ways to make sure that you do that without doing a bunch of math and speculation:
• If you are able to vape all day and not crave a cigarette so bad that you feel compelled to smoke one then you are probably getting roughly the same amount of nicotine as when you smoked.
• If you vape all day but you still crave a cigarette and you feel like you did when you tried quitting a hundred times before then you are LIKELY not getting enough (although there can be other reasons for this too - covered later*).
• If you vape for a few minutes and you feel nauseated and jittery and start to sweat then you are probably getting too much nicotine.
Your body will tell you what it wants/needs. Here are some other things to consider:
• Cigarette makers add something the cig to make it hit extra hard and to enhance the effect of the nicotine - it's known as freebasing nicotine (how scary is that). So when you vape, you will not have the sudden hit of relief that you get from a cig and in addition to that the nicotine that you get won't work as well - so it makes sense to get a little more to make up for that in the beginning. After a while your use will taper off and you may find that the strength you started with is now too strong for you - *once your body gets over all the other stuff in cigs. On occasion, people find that they are more addicted to the other stuff than they ever were to nicotine and that e-cigs don't help with that (e-cigs do have about an 80% success rate). Then you'll have to choose: go back to smoking, or use snus as a supplement to vaping.
• Nicotine by itself is not actually highly addictive - it's only when they add all the stuff in cigarettes that it becomes as addictive or more addictive than many illegal street drugs (you know which ones I am talking about). So even if you do end up using more in the beginning, it's not likely to make you more addicted as long as you are not experiencing the od symptoms listed above (nausea, sweating, jittery feeling). It's more likely that your use will taper off in time than increase.
GOOD LUCK!