use hfss is ok
In HFSS, you can obtain very accurate s_parameters.
However, it is very tough to extract the parameters you needed.
Some kind of equiv-model will be helpful, you can find them on MTT papers.
Translate s-param to Y-param and settle down every parameter one by one, then you will need some optimization tool such as Hspice to anneal the rest, inter-relating elements.
Anyway, this is only my own method. I am not very clear of that yet.
I will post it alone, maybe some one much clever will have their comments.