The basic idea is to counter the weight of the Z axis assembly so that it's essentially zeroed out. You don't want to overdo it, since the strut can work against you in the other direction if the motor has to fight to extend it. You do need to position the strut so that only its free range (7" in the one shown) is what's being used as the axis goes up and down. Here's how someone else implemented gas-strut counterweighting on a Taig: 12AMTools: Taig Mill Z Axis Air Spring Setup