Rp = (Rs^2 + Xs^2)/Rs
Xp = (Rs^2 + Xs^2)/Xs
This is in the ARRL handbooks. Just have to keep an eye on the sign of the reactance.
As W5DXP mentions, the impedance range is pretty limited. Above a few hundred ohms the errors get pretty excessive. For the high Z's I use old Gen Rad bridges.
