It was about 3/4 henna (the lowest dye content on the hennaforhair site) and 1/4 cassia, mixed with half lemon juice and half water.
I only left it on for about an hour and a half... I had some regular red dye left on most of my hair and didn't want it to go crazy. It blended in my roots VERY nicely (roots were an ashy strawberry blond, the rest was a medium auburn that had faded a little), and stayed within a fairly believable color
ETA: If you're thinking about henna-ing, I'd definitely order some smaller samples of the henna and cassia and do several strand tests (grab shed hairs from your brush or shower ). I think I had 4 (2 with the 1/2 and 1/2, left on for 2 hours and 4 hours, and 2 with 3/4 and 1/2, left on for 2 hours and 4 hours). It gives you a good idea of where to start, without risking turning into Ronald McDonald (that was my biggest fear too)