Octopus. Constant and variables

Hello, guys.
I wonder, is it possible to insert a constant value to the octopus optimization?
I have a form, and I need to get a minimum surface area within a constant volume by varying diameter and height. For now I only managed to get different examples, but they all have different values and proportion is quite linear - the more volume it has, the more surface area it gets. Could anyone help with it pls?