Of course, predicting taxes & Co. is difficult, as it will depend on thousands of things (as you can already guess from the amount of information you have to type into the Spiegel calculator linked by AlexR).
Rough guesses exist.
0th approximation: divide the gross by 2 and be happy if you get more net than this prediction tells you.
If you want a slightly more offical tax calculator than what Spiegel provides you with, the official software for income (and some other) tax declaration, ELSTER(*), does have a prediction based on what you enter.
Keep in mind that in Germany the difference between gross and net is much more than the income tax, as social insurance is calculated and deducted separately.
(*) note that someone at the tax office has humour: Elster = magpie, which in tales/fables is attributed to be thieving...