Inductors, Inductance calculation
The whole tutorial doesn't account for parasitic effects as e.g. parasitic capacitance which is relative big in multi-layer rectangular inductor.


  How to correctly mesure inductance please.



Do research what kind of parasitic elements a multi-layer rectangular inductor has, make a model of it and estimate the range of the value of each element
Connect the multi-layer rectangular inductor to a spectrum analyser
Using the estimated values of the parasitic elements and the results of the spectrum analyser, you can determine which element(s) is/are dominating and estimate/calculate the value
Simulate your model by using the values found in previous step and verify if it approaches reality (the result of the analyser) and iterate by tweaking the values in order to have a better fit for the real case