I think 10 seconds back is the best solution.
How the command itself says, it's an SOS-Command, like the last possible cmd to rescue yourself before using /r.
Back to last CP might be too easy, as the cmd somehow should be a little punishment. Otherwise, for example in slow and curvy races through small towns, the teleport to last CP could be abused, cause it takes more time to get out of some front porch, instead of just teleporting to last CP (right back on the street).