In NC, you would create a model and set the channel to 25. Call it Dimmer for this example
Set the model type to single color white. In 3.6 you can set the model brightness, I would set that to 100 to start.
Select your model you created (Dimmer) , use the color wash effect and set the color to white and use the slider to set the value. This should then control your RGB channels 1-24 if they are slaved to ch 25 via the hardware controller.
I would select the OUTPUT TO LIGHTS button and use the sliders until I got the effect I wanted, then save that effect setting as a preset.