The title of your post says "live rock, but the body of your post says "life rock". Are you using the CaribbSea purple Life Rock, or real, wet live rock from a good LFS or mariculture facility like Tampa Bay Saltwater. I'd recommended using real live rock, not Life Rock, which is just a dry rock painted purple.
I think the old rule was 1 to 2 pounds per gallon, but that depends entirely on how dense the rock is, and some rock are more dense than others. That's also for a really well packed tank full of rock, and I think most reefers now prefer a more minimal rockscape.
I would suggest getting a little extra, especially if using relatively inexpensive dry rock. This way, you can have a nice mix of rock shapes and sizes to make a nice aquascape. Maybe purchase 1.5 pounds per gallon, and expect to use slightly less, might be a good place to start.
Good luck with your build!