Welcome to MR. Always nice to see new members asking questions before jumping into it headfirst.
Depending upon the collection location of the rock, Fiji, Marshall Island, Tonga, Caribbean etc., will determine the amount of LR you'll want to use. Most people use between 1.25 - 1.75 lbs/gallon.
Some rock is more dense than others and it'll require more of it to fill the same space. The less dense the rock is, the more surface area it has and the more biological filtration you'll get from it.
swimmer