I've been self-employed for just over 15 years now. My rule of thumb from day one has been this.
If you send the IRS 20% of your *GROSS* business income each quarter, then come tax filing time you'll be fine. In the 15 years I've done it this way I have only owed the IRS at tax file time, two times. Both times what I owed was less than $500. Because my estimated payments got my total tax paid to within 10% of my tax liability, I've never paid any underpayment penalties either.
Note that if your state also taxes personal income, then I recommend you pay your state a percentage of your gross business income of whatever the maximum tax rate is. Then come tax filing time you're practically guaranteed a refund from the state. At this point in time the highest state tax rate I'm aware of is 8.75%
