Working out how much tax you need to pay can be a pain in the backside, and unfortunately it can get a little more complex than an % of income you earn - dependance allowance.
If you're looking for a general estimate, then I'd suggest using one of those Paycheck Calculators like the one here:
This is especially true if you are self-employed, as there are very often deductibles that you would have never considered before!
For example, you could pay $500 to your accountant, and they might come up with an additional $4000 you could claim back for various expenses. They are insanely clever like that!