You say, "I have no way of actually measuring it." But I wonder about
that. Certainly it's not too difficult to find the velocity factor
with the simplest of equipment, and with a bit of creativity, you
should be able to figure out the impedance as well. In any event, it's
pretty common to make at least minor tweaks on matching stubs to get
them to do the intended job. Will you have any way to know when it's
doing its job properly? That in itself may be all the measurement you
need.

Cheers,
Tom