The 20% mentioned is definitely NOT correct.
In Germany (at least at the moment) most shift workers gain certain tax advantages by working Sundays and public holidays. Smaller companies deal with this on an individual basis which means that your net income can vary quite a bit from month to month.
Larger companies such as Lufthansa agree an average figure with the tax authorities and I think this is where the confusion with the 20% comes from. The actual figure is 16.3% tax free. You will often see this mentioned on the pay scales of ppjn.com.
There are also important tax advantages available for those coming out of training. I know of at least one person who managed to pay no tax for three years as an offset of his training costs, which as the previous answer mentioned, shows the importance of getting good advice.
KT