Think the idea of putting anything like a small SMD cap, resistor or inductor on a FR4/Epoxy PCB to use as some "Reference" is unwise. PCBs of FR4/Epoxy or similar material change with humidity, temperature, time, handling and so on, some PCB materials are better tho, and maybe those would be acceptable.
Maybe others can comment about the different PCB materials, and the relative behavior for supporting precision SMD devices.
As mentioned above every mm counts when measuring nH, and that any equipment/cabling moves can cause measurement uncertainty, including Kelvin cables. This is why we don't use Kelvin cables for precision measurements of SMDs, even the Tweezer Kelvin types. We go so far as to do a Open Cal with an LCR SMD fixture by placing the plunger opening distance equal to the expected DUT SMD spacing, this helps capture the fringe capacitance with the plungers at the expected spacing, thus minimizing the fringe capacitance effects on the DUT measurements.
Along this line, for the Short Cal, we often use a Zero Ohm SMD of the same size/type as the expected DUT, altho not certain this is beneficial in all cases. The thinking here is that the Zero Ohm device helps compensate for the expected contact area of the SDM DUT as well as the plunger spacing and tension.
The SMD fixture plunger spacing is an interesting thought regarding inductance measurements (as well as the mentioned capacitance measurements). Since inductance is associated with length and the fixture plunger has a diameter of ~1mm (fixtures we have) and varies in length when engaging a SMD. Then a 1206 SMD that was considered a round "wire" has a self inductance of ~1.14nH (Grover's) (1.0938nH), an 0603 0.40nH (0.4983nH), an 0402 0.22nH (0.3059nH) (Edit: found a flat wire inductance estimate). So it seems that when measuring small SMD inductances doing the Short Cal with a Zero Ohm SMD might be useful.
Disclaimer we have no way of knowing if these inductors are accurate per OEMs data sheet, and did a very crude test with a small SMD inductor, 0603 10nH +-0.2nH (Wurth WE-MK type). We see a difference in the ball park as shown by Grover's estimate above relative to if the Short Cal is performed with a Zero Ohm SMD device or not.
Later if time permits we may expand on this crude test, but seems that the concept of utilizing a similar size Zero Ohm SMD for the Short Cal with a SMD fixture is worthwhile for small inductors, and spacing the plungers at the proper spacing for the Open Cal is also beneficial for small capacitances. Does anyone know if "Zero Capacitance" SMD devices exist? We've thought about just using very high resistance SMDs of various sizes for the "Zero Capacitance" equivalent, but haven't do so yet.
Best,