Using media length as the track seek value in a service

I am trying to make a next track service for my plex, as it seems the next track is unsupported - seeking to a value works, but I would like to use the length of the current track as the seek position

I have tried various configurations of the following as I have managed to find all the info in attr info and tested the seek function with a plain tect number.

service: media_player.media_seek
  seek_position: {% state_attr("media_player.floor_guy","media_duration") %}
  entity_id: media_player.floor_guy

does any one have any idea how this would work?