[xXOpkillerXx]

If you want to quantify stamina difficulty, I believe it is necessary to account for the position of the hard parts. When taken to the extreme you can clearly see that it makes sense: a 1h long file with the first 10 minutes being twice as fast as the rest will always be easier than a 1h long file with the last 10 minutes being twice as fast. The challenge is to find a non arbitrary factor for it.