Here is a derivation of the coupling of a pickup to another inductor like thing that I did a few years ago. The algebra is not so complicated, and I consider several limiting case to demonstrate that it makes sense, and then show the results in several forms. I have attached a png file for each page since I cannot figure out how to attach a multipage pdf file.
Edit: The pages can be blown up by putting them in another viewer if your browser cannot do it directly.