Yes, I get that, but on its own that piece of data is of no value.
What you need to know is whether, from exactly the same number of ewes, of the same breed, grazing the same fields, and with no changes in management, is he now consistently selling 12 more finished lambs per year than he was before you shot the foxes?
My guess is that he isn't, if all other things remain equal.
Some years will be better and some years will be worse, but in my (considerable) experience it usually takes a fairly significant change (or lots of little changes) to make a lasting reduction in mortality. Culling foxes barely scratches the surface, the effect is so minimal, but it makes people feel like they're doing something about it.