What if you've been smoking ultra light cigs?
That's actually pretty near meaningless.
Firstly, ultra light cigs are made ultra light by adding little holes around the filter (look real close and you can see them) which mix air in with the smoke. Thing is, those holes are usually where people have their fingers when they hold the cig, and studies have shown that people who smoke ultra lights have a tendency to hold the cigs in such a way as to block as many of the holes as possible. So odds are what you were actually inhaling was actually about the same as a light-side-of-normal cigarette.
Secondly, vaping differs from smoking in some important ways, including how the nicotine is absorbed in your body. The absorption rate with vaping is much lower and slower and varies more from person to person. So there's only a loose correlation between what you smoked and what mg level you should vape at.
I'd suggest trying at 24mg (where most people like to be) and adjusting from there. Personally, I vape at 18mg, which is the second most common dosage.