Ok this sounds crazy, I think you got something wrong in your formula because it just can't be...
Here the tax is about 40% of your
benefits (can be 50% depending on your status).
So let's say you made 100,000 last year.
You declare your expenses (accountant / book-keeper / invoices needed here...) that would be let's say 60,000 .
So your benefit is 40,000 so you'd have to pay about 20,000 in taxes.
And ::Xes:: yes when you receive money from a customer, you must put a part of it aside to pay for next year's taxes... Why would your clients not pay you when they get their invoice anyway? (just kiddin')