How much you need really depends on the density of the rock itself.. you might find you need anywhere from 1 to 2 lbs/gal
to get the type of aquascape you want. I'd probably go with 60-75 lbs and then add more if needed.
Once your cycle is complete (amm+trites=0) it doesn't really matter what the nitrate level is at. Do a big water change and then you should be ready to add a cleanup crew. If the nitrate level is still high after the water change then do another one before you add the critters.