Rule of thumb is about 1.25 - 1.5 lbs. per gallon. But alot depends on the rock density, and also what you want it to look like. As long as you leave room for the fish to swim, you can add as much as you want.
I've always been fond of Fiji....looks great. just my opinion
Do a search online to see some pictures of different types of LR...Fiji, Marshal Islands, Tonga etc,