I went down a rabbit hole trying to do this sort of thing, but found it much easier (and still accurate enough) to just track stove run time using history_stats.
configuration.yaml:
sensor:
# Track Pellet Fill
- platform: history_stats
name: Pellet Stove Runtime Since Fill
entity_id: switch.pellet_stove
state: "on"
type: time
start: "{{ state_attr('input_datetime.pellet_stove_last_filled', 'timestamp') }}"
end: "{{ now() }}"
climate:
- platform: generic_thermostat
name: Pellet Stove
heater: switch.pellet_stove
target_sensor: sensor.pellet_stove_zone_mix
min_cycle_duration:
minutes: 15
Then just create an input_datetime for the time that you last filled the stove. I do the same for the time I last cleaned it. Once you establish known values you can further abstract it into a percentage.
EDIT: a few more details:
switch.pellet_stove is a zigbee relay ($17.90) that connects/disconnects the external room temperature probe
sensor.pellet_stove_zone_mix is a sensor group that mixes the values of a few zigbee air temp sensors ($19 each)
