I think it's a good rule of thumb given that most people will be within an inch or two. My ape index is noticeably low and it's only -2.5". +/- 6" is absolutely

I think it's a good rule of thumb given that most people will be within an inch or two. My ape index is noticeably low and it's only -2.5". +/- 6" is absolutely massive. When I was a new climber I climbed with someone who was world class and just by watching me for a short time he could tell that my ape index was low.