Hi,
are you talking about
* mechanically adjustable capacitors, or
* varicaps
But in either case the formula: Zc = 1/cjw is correct.
You just need to know the value of C. If you adjust it to 20pF, then use 20pF in the formula. If you adjusted it to 30pF then...
Now you may say "but what if the capacitance is not known" ....
then you can´t calculate the impedance.
But the same applies to the frequency (or "w" in your case).
If "w" is unknown you can´t calculate the impedance.
Klaus