To calculate the impedance of a miscrostrip line you can find several applets for common configurations (microstrip above a ground plane...) on the web . You can have a look at the attached links.
Bare in mind Saturn or most of web based are simplified calculator only. Planarr solver is what you need. HyperLynx or Polar Instruments tools are industry's best. There is one open source(s) that gives the same (very accurate) results, but it is limited. It is here: http://mdtlc.sourceforge.net/