The self-employment tax is basically the employer side of your social security and medicare. It's 15.6% of your taxable business income. Note that the SE tax is "in addition" to the regular income tax you pay on that business income.
What I do, is just send the IRS 20% of my *gross* business income each quarter. Then come tax filing time I'm always well within $1000 of my total tax liability. So if I end up owing the IRS more, it's less than $1000 so no underpayment penalties are assessed. Typically, I get a refund. But in my 16 years of self-employment I've only had to pay additional to the IRS twice. Highest amount I had to pay was $472. That was nothing, so I just wrote the check and called it good.
