If you will not have any live plants I guess 1.5" gravel depth will be fine. In fact, it will be easier to clean your tank's bottom with less gravel. For a 75 gallon tank you'd need approximately 67 lbs of regular gravel in order for it to be 1.5" deep (assuming that the tank's dimensions are 48x18x20). Here is where I got this calculation from: http://www.plantedtank.net/substratecalculator.html
It is a substrate calculator that someone on Plant Geek gave me a link too. It's really cool. If your tank's dimensions are different than 48x18x20, just type its dimensions in the spaces provided on that website and it will calculate for you how much of the regular gravel you need.
I don't know why the two websites give different amounts of substrate needed for 1.5 for the same tank (48x18x20). But it's a good point you brought up. I think I'll ask that question on Plant Geek since I've been given links to these two websites there. The two calculators probably assume different types of gravels? (Not all gravels have the same weight + the grade of the gravel matters a lot too).