Azrael
circling Airstrip One
If they don't get 50%, is the election re-run, or are votes "redistributed"?A much shorter and cheaper option would be to use STV, where a candidate must have 50% to win the seat.
If it's the former, sounds much like the idea I support (except that my suggestion would lead to far fewer re-runs), if it's the latter, I don't believe your vote should count for anyone besides the person for whom it was originally intended.