Do harvest tests with hair from your comb or brush -- that's the only way to REALLY predict how a particular recipe will turn out on your hair.
Before my first henna, I did two harvest test: one with straight henna and one with 50/50 henna and indigo. The straight henna came out AWESOME in the tests and and the henna/indigo was way too dark and got even darker with time. Soooo, I took the plunge with straight henna on my whole head and I haven't looked back since. I love it.
I never thought the straight henna would work for me and I was concerned about going too red as my natural color is a dark brown with reddish highlights in summer. The spots where my hair turn reddest in summer is where I turn grey. Straight henna turns my greys a gorgeous dark golden red and the rest of my hair a dark reddish brown. So, I suggest whatever receipe you test, do one with straight henna, you may be surprised.
My recipe: henna mixed with warm water until thick mud. Cover and let sit at least 6 hours and I'm ready to go.
There are all KINDS of recipes and I've tried a few, but the best for me to achive great color and strength is warm water + henna. I aways do a moisturizing DC after rinsing out the henna. I use to put acids (ACV or redwine) in my mix for color release until discovered it is not necessary -- warm water and time is all that is needed for great color release.
Recipes galore at the following sites:
www.hennaforhair.com
www.http://forums.longhaircommunity.com/showthread.php?t=35188