I use Nestle's Nido which is available in Hispanic grocery stores in CA. The instructions say use 1/4 cup of milk and dilute to 1 cup with water. Since I usually use milk only for cooking or cereal, I usually just eyeball the measurements. If you actually drink the milk you should experiment and see what ratios suite your tastes.
My recolletion for Milkman 2%, which is no longer available, was that you mixed an envelope of milk powder with 1/2 quart of water, stirred, then diluted to 1 quart. I think I remember measuring out the amount of milk powder for a glass to 1/4 to 1/3 cup of powder with 3/4 cup of water.