I've read her book, but I don't use an acid in my mixes. Works great too!
IMHO, Catherine recommends too much henna/indigo powder for a given hair length. Depening on the consistency you choose, you can get great coverage for far less than recommends for X length.
I'm saying this to you cuz you gotta lotta hair, so how much do you use? If you're going by her measurements, that's like
500gms!