It never ceases to amaze us how much math is involved in cooking. You're looking to create something with a fat content of 10% to 12%, up from 3.5% for whole milk and less than 1/2% for skim milk. Heavy cream weighs in at 36% to 40% butterfat. If you use whole milk, mix 3/4 cup of whole milk with 1/4 cup of heavy cream to approximate half & half.
The proportions aren't substantially different if you use skim milk because even a small amount of heavy cream adds a lot of fat, but mixing 2/3 cup of skim milk with 1/3 cup of whipping cream will bring you within the right range for half & half.
If you use other creams with less fat - say, light whipping cream, at 30% - use more cream and less milk. If you use light cream, which averages around 20% butterfat, mix together almost as much cream as milk.
(Now if we can just get someone to check our math. There's never a kid around when you need one….)