I'm creating a parametric drawing of a cylinder and I've run into some issues with how the case inserts are dimensioned. The screenshot below shows the how the cylinder looks in its default format.

But when I increase the cylinder length it pushes the head insert upwards, instead of the case insert downwards.

How do I set the cylinder length dimension so it pushes the case insert downwards, whilst leaving the head insert fixed at the radius? Its worth noting that the number of fins increases with cylinder length which is why there are more fins in the second screenshot.