One way is to use a scene. Scenes allow you to set a light to what you want and capture that. If you look at the yaml for the scene you can see the values.
Another way is to set the color you want and look at the developer tools, status tab. The entity should show the values that should give you the same color. For me it says:
Thank you Edwin, I learned a couple of things there.
I tried adding the light parameters from the scene back into the data part of the above light.turn_on action but that did not work. Maybe scene.on and light.turn_on have different capabilities at the moment.
Anyway, I just activated the scene instead and it works well now, thanks again !
If this is the case that the parameters for light.turn_on do not produce whe same output that is weird. If so there is something going wrong with the integration. The current values for the light should be reproducible when set again. Maybe you should put in an issue with the integration owner explaining your case.
The only reason I can think of for too low brighness is confusing brightness (0-255) and brightness_pct (0-100) but you used the right one in your code example.
Yes, that is what I tried to say above (I even put in a developer tools screenshot), and what the OP says did not produce the right output. That is weird.