Oh the fun of mixing powders. Only way I know to lighten a powder paint is to use white. I have done this with some mixed success depending on the colors. Red , yellow, and orange went lighter. Black, purple, green, and most other dark colors speckled morte than it lightened them. All of the colors speckled and did not mix in a way other paints will but brighter colors did seem lighter even though it was just speckled. Adding clear might lighten them but you run the risk of speckled coverage with the head color showing through in places.

I have done it with many colors but never tracked how much to use so I can't help there.