I am practicing/ training my mind and eyes. Someone on another forum suggest this technique and I think it has merit ....
Go to a known area/ ridge or high trail. Using GPS ''goto'' a known mountain top, tower, ect that you can see. The GPS will tell you what the line of sight distance is. Study the details and train the mind and eyes to work together.
I did this today on a trail back. I was on a high trail on the side of a mountain and got to a fantastic vista where I could see to a mountain top that I have camped on many times. I guessed the distance from where I was and then checked my answer with the GPS. My answer was really close - but of course I know the area. I was exactly 3.25 miles from the object. I made a relation in mind to this number and what I was looking at.