physedgirl - I looked it up and Dr. Gordon has a lot of information on the BW formulas.
I found a page on the A-FNG and he does mention herxing.
Here is the link - it's very informative = has a challenge protocol and a treatment protocol:
www.gordonmedical.com/unravelling-complex-chronic-illness/services-offered-at-gordon-medical-associates/guidebook-for-the-byron-white-formulas-table-of-contents/the-byron-white-formulas-a-fng-and-chronic-fungal-infections/