Coupling effect - microstrip lines

Hi,
I want to design a coupled line along an existing circuit. I want a coupling effect of 10 dB for example.

Theorically, does it exist a mathematical expression giving the distance between lines versus coupling effect?

Thank you

As far as I can see, you are using ADS/Momentum do do your work.

Go to ADS, from the component palette choose "Passive circuit DG Lines -> MCline".

MCline stands for "microstrip coupled line". Just input requested data on element parameters and invoke the menu "DesignGuide -> passive circuit" and let ADS do the iterative maths for you.