isolating S-params in PCB board

[gf]

I know there is 2X method, 1x Reflect Method.
What method did he meant in his explanation?

I think AFR supports both flavors.

(BW is the frequency range of the VNA in use 43.5GHz for N5244B)
So, if you use N5224B. the minimum fixture  can be < 22pS

What makes you think that, when you follow the example calculation in the help text?

For example, a measurement at 26.5 GHz has a rise time of 37.7 ps. Therefore, the fixture length must be 4 * 37.7 ps = 151 ps.

1/43.5GHz * 4 = 92ps, or roughly 15mm, depending on the PCB material. Better use more than the absolute minimum.