I up voted this but I just wanted to follow this tangent.

as a dropout-cum-autodidact in a world where personal identity is not ontologically fundamental, I'm fractionally one of your future selves

This isn't true in all worlds where personal identity is not ontologically fundamental. It is a reasonable thing to say if certain versions of the psychological continuity theory are true. But, those theories don't exhaust the set of theories in which personal identity isn't ontologically fundamental. For example, if personal identity supervenes on human animal identity than you are not one of Warrigal's future selves, even fractionally.