Re: W(width) L(length) S(spacing) Coupled-Line Band-Pass Fi
The paper:
"Hairpin-Line and Hybrid Hairpin-Line/Half-Wave Parallel-Coupled-Line Filters" by Cristal, E.G. Frankel, S. Microwave Theory and Techniques, IEEE Transactions on
Publication Date: Nov 1972, Volume: 20 Issue: 11, page(s): 719 - 728
is a good starting point and has, I think, the design equations that you are asking for.
Dishal in the 60's wrote that mechanical filters didn't have to have resonators of different impedances, as the designs of the time implied. This made the process of filter design much simpler and his point is just as true with printed filters too.
see M Dishal “A simple design procedure for small percentage bandwidth round rod interdigital filters,” IEEE Trans. Microwave Theory Tech., Vol. 13, No. 5, 696–698, Sept. 1965.