Interdigital Bandpass Filter - Design procedure (Lancaster)

Hello,

I am following the design example of an interdigital filter with symmetric coupled lines as given in Microstrip Filters for RF-Microwave Applications (Lancaster), page 141.
I managed to calculate the characteristic impedances in even and odd-mode and, consequently, the coupling coefficients. My struggle is to calculate the spacing between ressonators. The author says the spacing could be calculated using design equations for symmetric coupled microstrip described in Chapter 4. However, I didn`t find any equation using coupling coefficient to calculate the spacing. Actually, there is no equation to calculate them. And it is bizarre because I spent a bunch of time searching for references that would show clearly this part of the design but no one expose this part. Even worse, the majority of the papers use this book as main reference to calculate the physical dimensions of the interdigital filter structure.

I believe someone has already been in the same struggle. Does someone can help me?
Thanks in advance!

If you have the time and the resources there is always parametric simulations using various FEM solvers. I once made CPW intergitial filters and the calculated line impedances, spaces, and gaps lengths were very different (10-15%) than the ones I ended up using for the final designs.