Solved!

Solution was to create a group and perform an action on that:

action: light.turn_on
target:
  entity_id: light.kitchen_cabinet_sink
data_template:
  brightness_pct: "{{100*state_attr('light.kitchen_sink_ceiling','brightness')/255}}"

Original:

Trying to run an automation to match one light's state (on/off/dim) to another's. Have this currently:

alias: Sync cabinet lights with sink light
if:
  - condition: device
    type: is_on
    device_id: [something]5710
    entity_id: [something]a438
    domain: light
then:
  - type: turn_on
    device_id: [something]b447
    entity_id: [something]470f
    domain: light
    brightness_pct: 100
else:
  - type: turn_off
    device_id: [something]b447
    entity_id: [something]470f
    domain: light

That works fine to turn the lights on or off, and I have triggers in the automation for that and changes in brightness. But using a non-static number for brightness_pct (yes, I know I'll probably have to math the 0-100 scale instead of 0-255) is giving me trouble. When I try something like this:

alias: Sync cabinet lights with sink light
if:
  - condition: device
    type: is_on
    device_id: [something]5710
    entity_id: [something]a438
    domain: light
then:
  - type: turn_on
    device_id: [something]b447
    entity_id: [something]470f
    domain: light
    brightness_pct: {{state_attr("light.kitchen_sink_ceiling", "brightness")}}
else:
  - type: turn_off
    device_id: [something]b447
    entity_id: [something]470f
    domain: light

I have also tried {{states.light.kitchen_sink_ceiling.attributes.brightness}} instead. Both seem to have the correct value when I play around in the developer tools. But when I put it in the automation, I get an error that a float value was expected. I see some similar issues online, but it always seems to be in a different context and people fix it by changing some value I never had.****