If you're pressed for time, Catherine Cartwright-Jones has it at her hennaforhair and mehendi sites. She's pricey, but fast. If you order now, you'll have it for the weekend.
I downloaded her free Henna/ "How To" ebook. It was very informative...I highly recommend it....here's the link
http://www.hennaforhair.com/freebooks/#
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!
After I cleanse w/ shakaika powder and air dry.
I use 1/4c henna mixed w/ water...then I mix 2/4c indigo,1T of salt & water...after each is mixed separately I combine them...to get more bang for my buck I put it on w/a plastic cap overnight....it's really helped w/strength & thickness. I have fine hair *heavy sign*