template:
  - sensor:
      - name: "Tank Level"
        unit_of_measurement: "%"
        state_class: "measurement"
        state: >
          {% set volts_full = 6.0 %}
          {% set volts_half = 4.0 %}
          {% set volts_empty = 2 * volts_half - volts_full %}
          {% set current_voltage = states('sensor.digital_gauges_tank_sender_voltage') | float(default=0) %}
          {% set increasing_voltage = (volts_full > volts_empty) %}
          {% if increasing_voltage %}
            {% set slope = (volts_full - volts_empty) / 100 %}
          {% else %}
            {% set slope = (volts_empty - volts_full) / 100 %}
          {% endif %}
          {% if current_voltage == 0 %}
            0
          {% else %}
            {% if increasing_voltage %}
              {% set level = (current_voltage - volts_empty) / slope %}
            {% else %}
              {% set level = (volts_empty - current_voltage) / slope %}
            {% endif %}
            {% set level = max(0, min(level, 100)) %}
            {{ level | round(0) }}
          {% endif %}