The types of studies you are referring to have been done historically. This is where I go back to square one: understand biology. If the plants are self-pollinated, you're going to have a different buffer zone than if they were cross-pollinated by bees. The zone for alfalfa will not be the same as the one for flax.
We've just interviewed people who seem to have a done a lot of work on this in the flax industry, and it's stimulated by the whole transgenic issue. It looks as if 40 metres would be a barrier for flax. It may be a little bit bigger for alfalfa, because you have to understand the biology of the bees and how far pollen is going to transfer. It may depend on whether you use honey bees or leafcutter bees. It's basically a science question; once that question is understood, you can come up with reasonable guidelines that will create isolations that are a compromise position for everyone.