Ingredient Ratios for Homemade Milk Chocolate
Like Sneftel said, whey powder is NOT milk powder. Whey powder is created using curdled and strained milk by-product. It's closer to cheese than it is to milk.

Milk powder is just evaporated milk.

With that out of the way, if I were to create a ratio by making modifications based on your ratio it would be: 30% sugar, 28.5% cacao butter, 21.5% cacao solids, 20% milk powder.

Again, make sure you are using actual milk powder, not whey powder.