On this Ft measurement, I think you should try to measure the full set of S parameters. This simply means turning the test set (whatever that currently is) around end for end and measure S22 and S12. If you were able to measure S21 properly with the 20 dB attenuator on CH0 and not overload CH1, then you may not need any pad on CH1. Your S22 values should give you a mag value close to UNITY. At HF, its going to be pretty high and the reactance will be -X or capacitive. On the mag of S12 in dB for any decent device even a 2N3904, you should see -20 dB or less reverse gain. Then go off and calculate Ft with the full equation.
FUN!
Alan
