So I need to come up with some saltpeter for this girl's witchcraft ritual, i've read that its possible to make it using ammonium nitrate and potassium chloride, aka the stuff in a cold pack and salt substitute. If I remember the reaction is

NH4NO3 + KCl KNO3 + NH4Cl

I need enough saltpeter that one could comfortably draw a 1 metre circle with it, am I correct to assume that I need an equal amount of both reactants and however much of either one I have is the amount of saltpeter I will get?