Pizza crust mix for pizza with cast iron pan?
So this takes a little bit of math.

You've got enough dough for a 12-inch diameter pizza. 

The area is π6^2 = 36π.  You don't actually have to do the rest of the math.

You have a pan that is 8-inches in diameter.

The area is π4^2 = 16π

To find the difference, divide 16π/36π and you get .44 (the π cancels out), which is a little less than half. For simplicity, I recommend making half of the box.

Be sure to use half of everything, particularly the water, so if it calls for 1/2 cup of water, use only 1/4 cup.