Yes it does have to be pi because that’s the formula for the volume of a cylinder. If you take a simple, cylindrical glass or container and measure it and apply the formula with pi, you’ll see that you’ll get the correct volume of the container. If you just want your kids to calculate a random x that’s 5* 10 *10 *10 just tell them to do that, don’t give them a made up formula, it’s not that difficult.