Gosh, I'm not really sure how much you'd need for waist length, but I'm sure Mehandi.com and HennaSooq have estimates somewhere. If you have a freezer, you can freeze left-over dye-released henna and it'll work once you thaw it...
I'm guessing you have 400 grams of henna and 100 of amla? Are you using the amla for curl retention or color?