Yep, that looks about right for 40lbs.
It looks to be dense rock though, what you ideally want is porous rock for the beneficial bacteria to be able to adhere to and for critters to live in. I would take a look at it in person, don't be afraid to say no if you go and the rock doesn't look great.
180g Reef - 60g Rimless Reef -90G FOWLR- 125g Malawi Cichlids- 40b REEF- 34g Fresh Planted-working on- 20L FOWLR- working on
I have a mix of many different saltwater fish amongst my tanks, but I love my Tangs most of all.