Brightness step not working properly?

So I have a setup that is using TP link bulbs. They work fine for everything but what I want to do is after sunset when a scene is selected have all the bulbs dim down to half of what they are currently at. I think I have this all set up correctly but for some reason the percent part is not working the way I think it should be and I could be wrong too. So my understanding is if I have this set to -50 It should look at the current brightness and reduce it by half. I have a master dimmer helper that is what sets all the bulbs. What is interesting to me and why I think its not working is if it is at 100 it will dim to 50… But if its at 10 it will go to 0 instead of 5. Or if its at 80 it goes to 30. I would expect it to go to 40 if it started at 80 so maybe im wrong in how this is supposed to work. Let me know what you all think.

id: '1784514482249'
alias: Dim After Sunset when Scene is selected
description: ''
triggers:
  - trigger: event
    event_type: call_service
    event_data:
      domain: scene
      service: turn_on
    enabled: true
    context:
      user_id:
        - blablabla
conditions:
  - condition: sun
    after: sunset
    after_offset:
      hours: 0
      minutes: 35
      seconds: 0
actions:
  - delay:
      hours: 0
      minutes: 0
      seconds: 5
      milliseconds: 0
  - action: light.turn_on
    metadata: {}
    target:
      entity_id: light.master_dimmer
    data:
      transition: 2
      brightness_step_pct: -50
mode: single

So It should activate a scene… wait 5 second for all the bulbs to get to where they should be then apply the -50% and do that over 2 seconds. I have played with delays and that seems to have no effect. So long as a scene is over 50% brightness it does not dim to 0 but if its below 50% it will dim to 0.

It’s additive, not multiplicative, so it will just add/subtract whatever percent you choose.

Ok that would explain why it acts the way it does. So then calling it a percentage is wrong and that’s what is confusing me. It even says it changes it by a percentage in the description. So what I will have to do now is figure out a way to read the current master brightness setting then halve it and set to the now halved value. Going to have flub around figure it out.

Any way that explains all that. A bit confusing but makes sense now. The description for Brightness Step should read more like it will move the percentage by this much from were it is.
Thanks.