I think his reasoning is quite flawed. He says that because we know nothing about our beginning and our end, that means we can't know where the middle ground is in how we go about living our lives. That's apples and oranges.

The nature of existence prior to birth and after death is about non-physical existence. To determine where the middle ground is in the way we live our physical lives, we only need to know where the extremes are in the way it would be possible for us to live our physical lives. The extremes of excess of sense gratification, for instance, as opposed to asceticism, or the extremes of excess of power versus powerlessness, etc.

Those are the extreme points that can be used to make the diagram. Whether or not there is any benefit to making such a diagram and living by it is another matter that isn't addressed in his theory.