how do I set the brightness to be the

[random isle]

You need #templates-archived for that

- service: light.turn_on
  target:
    entity_id: light.petunia
  data:
    brightness: "{{ state_attr('light.whale','brightness') }}"

[modest prairie]

I need to go study templates😅

[random isle]

The docs you need are linked from the topic of that channel