Im not sure if its the same with every product (bc of different brands and what not), but I do know that the stuff I purchased for my cactus, you just add 1 tablespoon/per gallon of soil used and mix it up..... If you already have everything going, you can just lay it on top of the soil and give it a good flush. Really, I dont think adding that much more can hurt anything after doing a little research on the matter.
Also, not sure where your located, but here in the STATES, I dont have a hydro shop in the state im in, so I purchased my bag at lowes for $8.00, it was a 2 pound bag, so should last quite sometime. Haha and dont get scared when you open it... it literally smells like a fire is burning lol.... scared the shit out of me because I opened it and layed it on my bed, came back in my room and almost had a panic attack.