Is fruit firmness the best measure to determine when to harvest apple fruit?

Apple flesh firmness is one criteria that is used to determine the maturity, and quality, of apples.  However, it is not the best single  indicator to determine the harvest maturity for apples.  Apples are harvested at different stages of maturity depending on how long they will be in storage before marketing. Apples to be used shortly after harvest are tree ripened and picked at a much later stage of maturity than apples that will be stored for 6-9+ months.  Flesh firmness does not reveal anything about the stage of maturity for making such decisions.  One of the better indicators would be the starch level or index of the apples.   As immature fruit develop on the tree they are composed of starch and as the apple matures the starch is converted to sugars.  Once all of the starch is converted to sugar the apple has no reserves on which to draw and the apple begins to senese.  So apples picked at an earlier stage of maturity will have more starch than those picked later.  The starch-iodine index is evaluated by cutting an apple in half and covering the cut surface with a potassium iodide solution for several minutes and the areas of starch in the apple will appear black and the level of staining can be observed.  Cornell University has a helpful harvest date publication that has a starch staining guide and solution recipe (which can be made at many pharmacies).