I cant remember exactly how much you are supposed to use, maybe one oz per gallon. Someone please jump in here.... But once you know how much laterite you need all you do is mix it with gravel to form a one inch layer on the bottom of the tank, then cap that with 2-3 inches of gravel. Thats what I use in my 75 and everything is doing wonderfull.
blackhole your right 1oz per gallon and thats kind of expensive its should be somewhere around 20 oz for $8-10 and 55oz for $14-16 look around online stuff is usually cheaper even with the shipping fee