Calculating line impedance take the distance of the trace from it reference plan e.g. VCC,VDD, GND as one of the factor influencing the impedance.

        My questions are:

        1 Which Reference plan it is best to refer VCC,VDD,GND "?".  What are the factors I should take into consideration ?

        2 How to refer to a trace between 2 reference plan e.g. VDD, VCC (not GND) ?

        3 Is there an educated article on that subject ?