In your model, how can you tell how close a market is to reaching the thresholds you suggest? In other words, if we are somewhere in the middle of converting a "large swath" of the population to vegetarianism, how can we tell if we're at a point of <1, 1:1, or >1 effect?

My guess is that we can't distinguish between those cases, in which case the best we can do is to average out over all long periods of time/market states and estimate that our long term effect is 1:1 (even though, in every case, it probably isn't exactly that).