Through training and still to this day, I keep on using these reference points to judge if I am at the right distance from the runway or the field I aim to land at (forced landing).
I suspect this is just a rule of thumb thing that works out just about right.
The geometry will have to do with the approximate height of your head vs the strut/aeileron, the altitude of your aircraft (when you commence a PFL) and the glide ratio of a typical trainer working out just about right to glide the distance to that point.
You simply do not have time to do complex calculations of glide range from the 1000-2000ft you would normally commence such an exercise. So a basic rule of thumb, that you can use by just turning your head, is as good an estimation as any. If it works, it works.