You are comparing weight verses volume. That would be hard to calculate. What type of substrate sand or rocks? 150 lbs of rocks will cover allot less than 150lbs of sand.
Please tell us the foot print dimensions of the tank and what substrate.
all depends on the look you are after and the footprint. I have over 100 lbs in my 57g because I wanted a slope rising to the back, I used 400 lbs to cover a 8x4 footprint with just a small amount so I didn't have to look at a bare bottom.