I deal with most varieties of domestic and some tropical fruits at work (no grapes however). Typically we will deal with Brix and TA. If I remember correctly BAR (Brix Acid Ratio) is simply the brix divided by the TA.
The lines may or may not cross in a graph (depends on how you set up the graph) but it is basically the same idea. Once you had a target BAR, you could repeat year after year.