The most recent census population (of the city proper; not the metro area) per Wikipedia.
divided by
The number of homes listed for sale in that city per Trulia on a given day.
I came up with it at work when I got too bored to do my actual job. I'm sure plenty of other people have thought to do it before, but as far as I know, they haven't trademarked it. So! I named it the Mapelthorpe index.
The index is designed to give someone a feel for how readily available housing is in a given city. There are obviously lots of factors that could sway a city's index number (e.g.; proportion of older or younger people in the population, lots of transplants from other cities buying homes, etc.). But in general, the higher a city's population, the more homes you expect to be for sale there.
The index isn't a perfect measure of how tight a city's housing market is, but it at least alerts you that something's up with the city's housing supply if its index number is much higher than the median range. And for the cities I picked the last time I did the index, average home price rank correlated pretty closely to the city's index rank. The city with the lowest index number (roughly 117) was Atlanta. The city with the highest number (over 1,200) was San Francisco. The median range seemed to be between 200 and 400.
I'll do another round of cities tomorrow and display the index numbers here.