Optimal χ² selection criteria applied to the ZDI solution set of HD 1237. Each panel contains the results for the July dataset using the Gaussian (left) and Milne-Eddington (right) line profiles. The red symbols show the behaviour of the total entropy content S_T as a function of the reduced χ². Each point corresponds to a converged ZDI solution. Green and blue symbols represent the first and second derivatives as indicated. A fourth-order polynomial fit has been applied (segmented line) to find the value for which the rate of change in the information growth (second derivative) in the ZDI solution set is maximised. The optimal fit level is indicated in each case.