Owen Duffy wrote in
:

A further issue is the accuracy of the estimate of the coil's
electrical length when represented as a transmission line. Using the
length of the wire in the coil (as is sometimes done) is too
simplistic. The Corum paper referenced at the calculator above
describes a method that appears to be more reliable.

I should have added... but what are you going to do with a more accurate
estimate of the equivalent electrical length of the coil anyway?

Owen