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For a collection of a modest number of target diameters for a metal product,
we have the setting on a machine that produces each diameter.
For an arbitrary diameter,  the "Lookup diam," not in the collection,
we would like to predict the setting that would produce that target diameter.
We use linear interpolation, sometimes also called a linear spline.
The "linearly interpolated" column gives the prediction for each interval.
The XLOOKUP function in the "Predicted setting" cell, chooses which formula to use.
See Excel documentation for all the options for the XLOOKUP function.