Explain refs more clearly

[flickerfly]

I read the docs and I don't understand what is used to determine a ref. I'm trying to play around with it on https://ergogen.cache.works/ default example.

I tried to change the points.mirror.ref to a variety of things like thumbfan_near_home and thumbfan_far_origin. Nothing in the thumbfan context seems to work. I'd like to judge the distance between mirrors based on the distance from the thumb key furthest to the left. Can this be done?