I know this question wasn't addressed to me, but just in case you might find it helpful: I've found that using a ratio of 4:1 henna/amla or 3:1, does result in my curls loosening, but only temporarily. To be on the safe side, I don't do repeated full-headed henna applications (no matter how much amla is added). Instead I do root touch-ups. To do that, I make a more viscous mix using less water than usual, to keep it from seeping onto a lot of already hennaed hair. There has to be some overlap when I do this. There's just no way to be as precise as it would be with commercial dye. But at least for me, the overlapping color is not at all obvious (I think).
Amla alone is a wonderful, albeit messy, treatment that brings out my hair's natural springiness. Sometimes I use it after suffering from over-conditioning, as it brings "structure" back to my hair. It's also an amazing facial treatment.