There was also an article in BYO magazine a couple of months ago (https://byo.com/article/brut-ipa/
) and a free article on the MoreBeer web site (https://www.morebeer.com/articles/a_brut_ipa_tale
). Adding the AMG during the mash does not give as low an FG as adding it to the fermentation, bottoming out at 1.004 - 1.006. Adding it during fermentation can cause problems with diacetyl, though. Adding the AMG at SG of around 1.020 seems to be the best. However, adding it during fermentation may leave a beer with little foam retention because all the dextrines are gone. The inventor of the style suggests adding it during the mash, then adding some DME at the start of the boil (after the AMG is denatured) to give some good foam, although you will not get to 1.000 FG that way.
I am trying to decide which is more important to me: foam or a few less points in the FG. I am leaning towards foam, myself.