Half and half is a mixture of milk and cream, resulting in a butterfat content of 10% to 12%. So a combination of your pure cream (what we call light cream) and whole milk - perhaps just a little heavier on the milk - will produce the right mixture.

In an online search, we came across Pauls Thickened Extra Light Cream - widely available in Queensland; less so in New South Wales, Victoria, A.C.T., and South Australia - with a butterfat content of 12.5%. Perhaps you can track that down in a supermarket and save doing all that complicated math.

*The first paragraph works for the rest of us, too.