TRL calibration reference impedance is the TRL line impedance even if
the calibration result z0 is by default 50 ohms. Using scikit-rf
NISTMultilineTRL for 70 ohm line you should specify z0_line = 70. z0_ref
should be either 50 or 70 depending on if you want the resulting
S-parameters to be referenced to 50 or 70 ohms. NISTMultilineTRL also
has ref_plane argument that can do port extensions accurately with the
solved transmission line propagation constant.