Here is a milk-math chart for you (and for me):
1 gallon milk = 2 & 2/3 c. powdered milk + 1 gallon water
1 quart milk = 2/3 c. powdered milk + 1 quart water
2 cups milk = 1/3 c. powdered milk + 2 cups water
1 cup milk = 3 T. powdered milk + 1 cup water
1/2 cup milk = 1 & 1/2 T powdered milk + 1/2 cup water
1/3 cup milk = 1 T powdered milk + 1/3 cup water
You may also want to print out this cute powdered milk conversion chart created by Crystal at Everyday Food Storage. You can hang this on your fridge or tape it to your can of powdered milk.
Note: Check your milk containers to compare conversions. Amounts to use actually vary by brand (and by non-instant vs. instant).