They calculate by taking your TOTAL salary income in the tax year - it's not based on monthly amounts as such.
As this document shows: http://www.ird.gov.hk/eng/pdf/pam61e.pdf
Your income chargeable to tax is your total income in the tax year MINUS your allowances & deductions. In the simplest case as a single person with no other deductions your total allowance is HK$100,000.
So in your case for 2008/9:
Total income = $175000
Allowances = $100000
Net chargeable (i.e. taxable) income = 175000-100000=75000.
The first $35000 of taxable income is taxed at 2%, which is $700 of tax.
The next $35000 of taxable income is taxed at 7%, which is $2450.
The next $35000 of taxable income is taxed at 12%, but in your case you only have $5000 of income in this band, so that's $600 more tax.
Total tax bill for the year: $700+$2450+$600 = $3750.