You usually want the inductance per turn of the given core, Ln usually called. Thay way you do L=n^2*Ln
To find it having the core you do 10 turns or 100 turns and do the inverse, usually better than just relying on a single turn measurement.
If you don't have the core you can sometimes find the value somewhere or you can do the math as you are doing, but having the correct μr and length is usually tricky. Also the μr could change with signal amplitude, freq and other factors, so experimentation with the actual conditions is the best way to find the value.
JS
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