HFSS - trying to find S-parameters of an open N plug at reference plane

I'm looking for a cheap way to make an "open" standard for a VNA calibration kit. One way is to just leave the N plug open. Agilent's FieldFox range of portable VNAs have an optional "QuickCal" which uses the RF connector as an open termination, with no need for a mechanical caibration kit. You just have to tell the VNA what connector you have (N, 3.5 mm, APC-7, 7/16 etc) and what sex it is. This is not as accurate as a mechanical calibration kit, but is often good enough.

Obviously the N plug is not a perfect open - it is impossible to make a perfect open circuit. There will be fringing capacitance.

So my aim was to model the N plug in HFSS, and then find the how the fringing capacitance varies with frequency. I intended drawing the N plug as a bit of air-dielectric 50 Ohm line, which tapers towards the end, like a male N plugs inner conductor does.

Is there a way I can put a waveport on one end of the coax, but find the S parameters at the reference plane of the N connector, rather than where the waveport is?

I believe one of the assumptions when using a waveport are that the fields have "settled down" so

How long should the bit of coax be in order that I can accuratley predict what would happen at the reference plane? If the length of the coax was say 20x the diameter, would that be sufficient?

BTW, for the N connector, the reference plane is defined as the mating surfaces of the outer conductor - see http://www.cmi.cz/download.php?wdc=1438
Deborah