Maarty, Here's the short response to your concern .....
Income $100,000 (As computed on your SCHEDULE C and added to your 1040
Standard/Itemized deductions $6,300 (As shown on your 1040)
Personal exemptions $4,000 (As shown on your 1040)
Taxable income $89,700 (As computed on your 1040)
Estimated tax liability $18,219 (But add an approximate amount of $1,000 to pay for your SE Tax))
So a self employed person with a total income after deductions of $100,000 as computed on his Schedule C with no kids would pay about $20,000. (20% of his total income)
You would need three Federal Tax Forms to compute your 2015 tax obligation,
1. Fed Frorm 1040
2. Schedule C
3. Schedule SE
Pretty easy and FUN once you get the hang of it and if you are in business for yourself it's pretty damn necessity