Choosing is a subjective matter; or at least how we choose could be the subject of a lengthy philosophical analysis. Perhaps that is why so much of the literature ignores this question. Without any firm theoretical basis, I chose to be approximately the noise level. I estimated this as follows.
The simplest method of choosing is to find the average v2 in the plane and then choose some arbitrary fraction of it, say 10% of the average. Although this method worked in Figure 2, I prefer another. I chose to be the median value of v2. (In other words, we conceptually prepare a list of the numbers v2; then we sort the list from smallest to largest; and finally we choose the value in the middle. In reality, median calculation is quicker than sorting.)
Notice that Figure 2 uses more initial crosstalk than Figure 1. Without the extra crosstalk I found that the first iteration worked so well, the second one was not needed. Thus I could not illustrate the utility of nonlinear estimation without more crosstalk.