in general they will advice you to use 100g/  12", so you'd need around 200 - 250 g. but this is not a rule. the thing is, if you have medium brown fine hair (like me), you have "excellent" hair for henna, since it's the type of hair that will take color the easiest and quickest. i've found that those measurements are ok for a first application, but way too much for touching up so nono, henna doesn't mean expensive! but remember, that site has rather high shipping cost is high (i took the second most expensive option), so you better order for a whole year if your wish is truly to go off chemicals and keep up with henna (especially since after two applications it's pretty more premanent on my hair than the hair dye i used before).
secondly, do you want soft black or jetblack? they have a very nice soft black in their premixed herbal henna's, i use it mixed with their dark brown and obtain a color my friend likes to call "warm black". i call it a very dark cool almost black.  8)  it looks very natural.
but if you want a jetblack you might be better off with a first step of body art quality henna, followed by a step of indigo (some people like to mix the indigo with amla and sea salt for better staying power, but they don't sell pure amla on that site). i think you'd get more "even" results as well with the last method since pure henna is WAYYY nicer to apply than their herbal henna's, which are very grainy and seem to cake quickly to the hair, which means you don't get it everywhere! indigo is a very runny mess, that's why people prefer to mix it with another herb like amla, or a neutral henna (cassia obovata).

i am kinda lazy, and i am not interested in a jet black so i use their herbal henna's (i am getting better every time at finding tricks to use less and have more even results), i also avoid having to do two steps everytime. but then again, i henna every month, while you could as well do it just every two months with a double process.
the first time i henna'd, i used ahmad (pure henna) on my length (i wanted especially to avoid having to redo my length too often), and used the herbal on over it, to be sure i had a really good coloring and staying power. i only need to put a very small amount now on my length, which i dilute with conditioner and their lotus powder plus (smells really nice!).  i dont think i would have to redo my whole length ever. i typically use 80 g to touch up my roots, and put the left overs on the length.