Being in the military it comes as a sort of second nature mate.
Using known distances to assist in rangefinding.....
Just think of a football pitch.....from goal line to 18 yard box, double it brings you to the outer ring of the centre circle, add 15 brings you to the centre spot.
So when you're in a field pick a point in the distance and imagine yourself stood on the goal line, draw in your mind half a football pitch, split that into your outer ring and 18 yard box and that will more or less give you a basic idea of distance!
However I know Si(Zini) uses both a gun mounted laser an