The amount of coupling is dependent on the electromagnetic field surrounding the conductor. The amount of EM field depends upon the amount of current or charge flowing through the microstrips lines. If there is no current then the coupling is zero. Hence coupling is dependent on the current amount and its rate which in turn depends upon the frequency and other factors.
But in board sense there is always finite amount of coupling.
I think Ronv is referring to the parallel capacitor formula. If so I think this would not be accurate in microwave frequency domain.
I want to calculate gap distance required between two microstrip for minimum effects on each other, which i believe is the term coupling.
There are so much advanced formulation in textbooks regarding this problem and the topic is broad itself, so I wanted to ask designers here what formula is used in my case.