Molecules angle to z axis using gmx gangle

Hi all
I am trying to use Gangle module for calculating angle between molecule and z axis.
i guess gmx gangle can be used for that. The issue is molecule being symmetrical rotates around its principle axis as well. so i am having trouble defining atoms for vector. looking forward for suggestions from the group.
I am not very clear about how plane selection works? can i use plane selection for the purpose?