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I'm not a tax guy, but based off my experience:

Assuming he's single: Federal taxes will be around $47.6k. (~28% at $170k, the 33% bracket hits you at $171,550.) State taxes vary widely. Some cities even have income taxes (e.g. NYC). Next you would have to add in investments, interest on investments, things like that as your income. Then you've got to consider deductions from your home, education, etc.

So it's a bit more complex than a simple percent; however, I would budget for a 35-40% effective income tax rate ($59.5k - $68k) in my state; however, in my state the cost of living is about half that--so taxes would be off $85k instead of $170k.