I have over 250lbs of rock in my 125g 6 foot tank and let me tell ya, it's ALOT of rock LOL.
I plan on taking out about 75lbs or so to move it to my new 90g setup leaving approx. 175 lbs which I think is a nice amount. I'd get at least 100lbs or so to start, between live rock and base rock if youre on a serious budget. You can add base rock as you go without a problem so you can drop $100 here and there til you get the amount you want and like.
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.