Setting Brightness From Template Sensor?

Configuration
2

If you are starting out in HA, you should use the current format for template sensors instead of the legacy format, and you should include defaults where necessary

The following is working for me. One thing to check is that your Hue devices support using brightness_pct… not all devices support all the configuration options available in every service.

template:
  - sensor:
      - name: "Sun Elevation"
        unit_of_measurement: "°"
        state_class: measurement
        unique_id: "sun_elevation"
        state: "{{ state_attr('sun.sun', 'elevation') }}"
        availability: "{{ state_attr('sun.sun', 'elevation') is number }}"
      - name: "Circadian Brightness"
        unique_id: "circadian_brightness"
        state_class: measurement
        unit_of_measurement: "%"
        state: >
          {% set max = 0 %}
          {% set min = -10 %}
          {% set current = state_attr('sun.sun', 'elevation') | float(0) %}
          {% set percent = (((current-min)/(max-min)) * 100) | float(0) %}
           {% if percent > 100 %}
             100
           {% elif percent < 1 %}
             1
           {% else %}
             {{ percent }}
           {% endif %}
        availability: "{{ state_attr('sun.sun', 'elevation') is number }}"

FWIW, you can compact your if/then into a single line using sort():

{{ ([1, percent, 100]|sort)[1] }}

alias: "Hue Brightness"
description: ""
trigger:
  - platform: state
    entity_id:
      - sensor.circadian_brightness
condition:
  - condition: state
    entity_id: light.bedroom
    state: "on"
action:
  - service: light.turn_on
    target:
      entity_id: light.bedroom
    data:
      brightness_pct: "{{ states('sensor.circadian_brightness') | float(0) }}"
mode: single