Compute loss/inch for (differential) T-line from s-parameters?
S11 is input return loss. S22 is output return loss (output driven and reflected energy measured). S21 is forward gain, input driven and output power measured. S12 is reverse gain, output driven and input power measured.

You have various measurements, including S21 for 2 different lengths, for various frequencies. Did you normalize the S-param measurements, before measuring the PCBs? Normalization allows de-embedding the item-under-test from the cables and SMA connectors. If you did "normalize", can you tell the difference?

Suppose at 1GHz, for 1 meter the S21 is -0.5dB, and for 2 meters the S21 is -1.2dB. Does this mean that PCB, at 1GHz, has 0.7dB loss per meter? I think so.

[EDIT Read Mike's discussion below, in the comment. ]