Gravel under concrete stairs
For the most part, the slope the gravel naturally lays at when set in place will tell you how much concrete to order, not much you can do about it.

But, if you are really wanting to cut down on concrete and use more gravel, and you have total control over the pour, you can start at the bottom. Fill the first tread and start adding more gravel as you go, adding it to the top and letting it slide down, making sure you keep the reinforcing wire or rebar up out of it. Do make sure you keep a decent amount of concrete in the inside corners of the stairs, (3-4") so the stairs don't get weak.