24 red rings isn't outside of the possibility for annulata, you need to keep in mind that the range in Markel's books is a generalized average range of ring numbers that doesn't include aberrant patterns....It is really a guide on what a typical snake could look like not a definitive guide. If you want to confirm whether or not it is an annulata, you need to do a scale count or figure out where you can get a DNA comparison run.
I should also note that other guides (such as this one by the famous Hobart Smith) do not count red rings but white/yellow and from what I can tell, your snake is pretty close to being in the range (at least not extremely outside as the red bands indicate), which could easily mean a slightly aberrantly patterned animal.
see
http://sierraherps.com/pdf/Smith[1].1942.pdf
Smith, Hobart M. "Remarks on the Mexican king snakes of the triangulum group." Rochester Acad. Sci 8 (1942): 196-207.
If you want to go with the definitive book on milksnakes and how to identify them correctly I suggest Systematics and natural history of the American milk snake. 2nd ed. Milwaukee Pub. Museum. While Markel's book is good for a TFH pet book, it lacks more than a little as a definitive work.
some comments
Ed