Potato powder will work fine, your problem is that 1 cup of potato powder is not the same as 1 cup of potato flake. If you run one cup of flakes through a blender, it will take up less space and will probably be closer to 2/3 to 3/4 cup of powder. I usually run my recipe through a blender for a better mix, so while I have not actually measured the difference, I think that my example should get you pretty close.
So if you are doing a recipe that uses 6 cups of potato flake, you would instead figure out, lets say, 3/4 cup x 6. That would be closer to 4 1/2 cups pototo powder instead of 6 cups of potato flake. Again, it's not exact, but I think it would be close enough to get production.