Yes, it isn't a completely powerless dropper but far more efficient than a resistor in most cases. Substitute Xc for R to calculate the value you need.
Words of caution though:
1. The value of Xc depends on the frequency. Xc = 1/(2*pi*f*C).
2. If it fails short circuit the full voltage appears at the load.
3. Unless you have some DC present, you must use a non-polarized capacitor.
4. If you disconnect the source or the load, the capacitor might retain charge.
5. If you are using it as an AC line dropper it provides no isolation barrier could be extremely dangerous.
6. If you are using it as an Ac line dropper you must draw equally on positive and negative half cycles or it will simply charge up.
Brian.