If you go to hennaforhair.com, there are a bunch of mixing suggestions for every color and combination under the sun.
For me, I knew I didn't want a LOT of color, so I opted for the lowest dye content henna, and decided to add some cassia (adds shine, but not much in the way of color) to 'dilute' it some. As far as how much of each, I made the best guess I could from what the hennaforhair site said, and then did strand tests. I picked the combination I liked best (3/4 henna and 1/4 cassia for 2 hours) and that's what I put on my hair
"How much" of the overall mixture you need depends on your hair, it's thickness, etc. I'd check out the website, there's all kinds of info on that stuff. the '3/4' and '1/4' is literally 3 parts henna to one part cassia.
CG/Mod CG (soap bars) since 8/12/08
CO wash/Cond: Kathymack & Flowermoon Castille soap bars, V05 Chamomile Tea, Suave Ocean Breeze, Biolage Conditioning Balm
Styling loves: DIFFUSING! CK, KCNT, KCCC, FOTE (on dry hair only), Re:coil, Proclaim gel
Not sure about: Boots, Tweek, KBB Milk, DMHJ
HATE: plopping, FOTE on wet hair, BRHG