Here's my suggestion for solving this:
1. Get the client to send a position update when the player clears the red area.
2. The server calculates whether the player takes damage when it receives the first position report after the cast completes (obviously there would have to be a timeout on this, if no position update is received then the last known position is used).

This does not require major changes to the software or put more load on the server like reducing the update period to 0.1 second would.