The precise determination of the thickness of the coating is dependent on the type of product and the coating material used, but in general, for this, you first need to carefully examine the results of the research, similar to your own test, to obtain a range of proportions corresponding to the thickness of the coatings, and To increase accuracy, design a similar test and use the results for the original treatment.
I agree with above. You must provide a cross section, then impaint according methods. Take some picture with microscope. Finally, measure the thickness of the fruit cuticles.