Half and Half Substitute: Just Mix Milk and Cream?
From the Joy of Baking (although the term "half and half"gives it away):
Cream, Half & Half (10 - 12% Butterfat) 1 cup (240 ml)
7/8 cup (210 ml) whole milk plus 2 tablespoons (25 grams) melted unsalted butter
1/2 cup (120 ml) light cream (5% butterfat) plus 1/2 cup (120 ml) whole milk
1/2 cup (120 ml) partly skimmed milk plus 1/2 cup (120 ml) heavy whipping cream (35%)
Read more: http://www.joyofbaking.com/Ingredient...
None of those will get you into the 10-12% range. And no form of light cream is 5% butterfat -- that's basically whole milk. The first formula gets you around 15%, the second (corrected to 30% milkfat for light "whipping" cream) puts you at about 17%, and so does the third, without really knowing what "partly skimmed" means. If you correct the second to 18% milkfat for light "table" cream, then yeah, you're at about 12%, and then we can take the term half and half literally.
Obviously not your fault; the website is dead wrong.
But I'm just being picky and pedantic; ultimately it doesn't matter much. If it's a bit richer, that can't be a bad thing.
Half and Half, by law, is about 10% milk fat minimum (and the minimum is what you usually find in stores) but it can legally be up to 18%. Heavy cream is, I think, 36% milk fat, so somewhere between 2 to 1 and literally half and half Milk to Cream should get you close. I get 40% Heavy Cream from Costco so I go 3 to 1 when I need to make Half & Half.