Cream, milk and half & half

Diana B, I use a Pearson's Square (https://thesolitarycook.wordpress.com/2012/02/01/lets-bake-bagels-part-1/) to determine how much of 2 ingredients needed to yield a third, in this case milk and cream to create half and half (typically 12% fat), which as has been mentioned is not actually half and half of anything. If you have whole milk (4% fat) and heavy cream (40% fat), you would need 78% whole milk and 22% heavy cream. If you have 2% milk, you would need 74% milk and 26% heavy cream.

Just for fun, here is a breakdown of fat levels of dairy products:
Half & half: 12%
Table cream (also known as light cream, and both are pretty obscure any more): 18-20%
Whipping cream: 35%
Heavy cream: 38-40%

I've seen half-and-half listed as being 12-18% fat (which is quite the range). I view it as equal parts skim and heavy cream. I've made a spreadsheet that will calculate out mixes of whatever dairy I have on hand to get me to a fat equivalent of what a recipe calls for.
Unless your recipe is going to be sensitive for chemical or structural reasons, like needing to be whipped, then I think you'd be fine to try it once and see how it goes. I often substitute in less fatty options in custards, but I wouldn't try it the first time before something special, in case it didn't set as expected.

The recipes in question involve making a custard; I have used half & half and even buttermilk in them with satisfactory results, but I just wondered if I was missing something. Thank you all for your responses!

I do it all the time. If fat is not an issue in the dish, then it doesn't matter.
I have even added some soy milk to cut the dairy load.

It's not a 50-50 ratio. Whole milk is approx. 4% fat, heavy whipping cream is 36-40% butterfat, and half-and-half is 10.5-12% fat. A 50-50 mix gets you about 21% fat. A better mix would be 1/4 heavy cream to 3/4 whole milk which puts you around 12.5% fat. (If you have plain whipping cream, which is 30-35% fat, you'll be at about 11% fat)

Sounds good to me, especially if it makes your grocery shopping easier.