What is your desired sand depth? Measure your tank back to front for depth and left to right for length then use the sites hand sb calculator http://www.aquariumadvice.com/calcs2.php?type=sanddepth
. I am assuming you are just starting to set the tank up, right? If so IMO
I think 30lbs if it gives you the look you want will be fine it will provide are for you nitrification process and the 40lbs of lr
would be plenty to do the rest for that size tank. If it looks good to you Id say it's enough. Maybe other AA members have their own opinion.