I think maju's advice to get a transmission line caculator first is a very good idea. TxLine and AppCAD are both good transmission line calculators that provide coupled line models. That will get you close to a reasonable result.
To test the result for higher accuracy, you can get a free 3D planar EM simulator to build and test it. I would probably recommend Sonnet Lite, since they have really good ports, very deep dynamic range and are by nature a shielded domain simulator which is good for modeling striplines. In Sonnet Lite you can get the odd-mode impedance and S-parameters by setting the ports to 1 and -1 on one end and 2 and -2 on the other. Then, to get the even mode impedance and S-parameters, set the ports to 1 and 1 on one end, and 2 and 2 on the other.
Using this method should result in very accurate structures.
--Max