It is actually very easy: In the prediction profile grid you Alt-click on the factor. A little window appears, where it is possible to select "Lock Factor Setting". Then the factor is fixed, and the dashed red line becomes solid. When "Maximise desirability" is then chosen from the red triangle menu this factor is not varied. More than one factor can be locked. An alternative solution is to select "Maximise for each grid point" in the red-triangle menu. Then all combinations of desirability is shown. You can then sort after the "desirability" column, to see which setting is best.
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