WTA should follow the spread. Making a liquid with no nicotine would not be WTA as the "W" in "WTA" stands for "Whole". I can't imagine anyone with a liquid containing only the minor alkaloid portion of the alkaloid spread... I know I couldn't make it and I wouldn't want to vape it.
The 30 mg WTA, which is pretty potent stuff, is 95% (28.5 mg/mL) in nicotine, and 5% (1.5 mg/mL) in all the other alkaloids combined.
You must not assume that the mg strength of WTA refers to just the minor alkaloids, it refers to the total alkaloid concentration, I.E. nicotine plus all the minor alkaloids. While WTA is mostly nicotine, the 5% other alkaloids is where the magic is.
edit:
To add something, one might ask, "Can't you then start with just nicotine and add the 5% other alkaloids to get WTA?". The answer is "No" for two reasons:
1. When you extract tobacco, you get 95% nicotine and 5% other alkaloids. Very sophisticated techniques could be used to separate the minor alkaloids from the nicotine, but this would be extremely involved and astronomically expensive. Far easier to extract WTA and simply dilute with PG/VG.
2. When you start messing with the product and changing it, it is no longer WTA. I define WTA as the
UNMODIFIED alkaloid spread extracted from whole tobacco. I can do that 'cuz I created the term and I get to define it.