[gumby]

A friend that was in Viet Nam ( West Point engineer ) used this and said it worked very well...I guess if you were lobing morter rounds
https://blog.outdoorherbivore.com/wilderness/estimating-distance-with-your-thumb/#:~:text=To%20calculate%20the%20distance%3A,or%20a ligns)%20the%20distant%20object.&text=Do%20not%20move%20your%20head,the%20other%20e ye%20is%20open.