There's no universal equation. A lot of unsold supermarket food here goes into a scheme that sends it to shelters and charities, for instance and quite a bit of the frozen vegies come from the other side of the world (universal seasonality!). What you need, though, is an equation for food miles as well as one for distribution and waste reduction.

I know the theory - but I've never been able to apply it satisfactorily. When I can. I buy from a farmers' market (ours is like an organic supermarket, but fresher and cheaper and the money goes straight to the farmers - I can only go when I get a lift from a friend, though, so some years it's often and others, very seldom), but beyond that, I just make my best guess and, when my best guess fails, I buy the cheapest, since money is a big issue in my life currently.