The spread sheet program "matching network calculator" which is available for free on this page:http://www.rfshop.co.uk/blog/downloads
can do what you need.
For example, it will convert the reflection coefficient
Mag =0.35 Angle = 78 degs , in a 50 ohm system, into an impedance of :
Does that sound like the right answer?
Incidentally, the program uses the same equation you mention. Maybe your problem was in the manipulation of complex numbers?