Before I quit smoking I smoked 50 cigs/day (that I admit to, anyway). 50/0.8 = 40mg of nicotine INGESTED. That would require 80mg DELIVERED via eJuice (per the 50% ingestion rate of the computation in the link). At the 3mg concentration I vape now (or have been until a few days ago) I would need to vape 80/3 = 26.7ml of (3mg) juice to maintain the nic level of my smoking days. And to be honest, I may have smoked more like 3 PAD but I don't like to admit that even to myself for some reason. So the 2.5PAD is very conservative.
I think I am currently vaping around 15ml, 20ml tops, per day. Compared to 26-30ish equivalent when I was smoking. So I figure, if that formula is correct, then I have reduced my nic usage, but not by a huge amount.
If you read Rolygate's explanation in the survey thread linked above, he says the ingestion rate can vary from 20-80%, which would make any estimates fairly meaningless because the potential margin of error is so great. But if you give a human some numbers to crunch, he will crunch them, regardless if they have much actual meaning in terms of accuracy

. IOW, at a 20% ingestion rate, I would have cut WAY, WAY down. But at an 80% rate, I would have increased it significantly. And there is no way for me to know, short of buying very expensive blood tests just for curiosity. Which is what Topwater Elvis effectively did (probably not voluntarily), which is why I was interested in his comment.
I wish I could remember more clearly what I vaped when I first quit smoking. I know it was 24mg for awhile, and I *think* it was around 305ml per day because I was going through a 15ml bottle every few days. From my calculation above, I arrived at about 80mg of eJuice nic equivalent per day when I smoked. At 24mg concentration, that is about 3.x ml per day. Which is the low end of the 3-5ml per day that I *think* I was vaping back then. So the numbers seem at least reasonable to me.
Hope this helps and doesn't just muddy the waters further...