The monthly quellensteuer amount should come straight off a table issued each year the canton of your employer and which are available on-line so you can check for yourself.
Since it is calculated monthly and not cumulatively (eg like UK PAYE) you should never have a situation that results in all of your income going. Unlesss of course they corrected an error of previous months.
You do have to pay some extra social taxes.
Here is the link to the ZH canton table. If you are single this is the table for 2007
http://www.steueramt.zh.ch/html/steu...if_a_QSt06.pdf (read the first column if you have no kids and the version without kirchesteuer assuming you declared no religion)
0.5% is correct for salaries in the month upto 1700chf. To be taxed 15% you would have to earn 13000chf in that month.