Making milk paint, much like mixing chocolate milk, doesn't require exact measurements. If your paint becomes too thick or runny, just add a bit more water or milk paint powder to achieve the right consistency. As a general rule of thumb, add equal parts milk paint powder to hot or room temperature bottled water (cold water will create lumps), and stir well! Mix your paint in small batches — you can always add more.
Tip: Because milk paint is nontoxic, it’s safe to use a handheld mixer or blender to combine the paint and water (which is the easiest way to remove any small powder lumps in the paint solution.) I used cheap dollar brushes to mix my milk paint, though, which worked nicely.