The estimated taxes on IRS Form 1040-ES are just that.... estimates. Those numbers are based on the tax return you filed, which was used to generate those forms. You do not have to pay what is on the 1040-ES if you don't want to. You can pay more if you want, or less if you like. You can even not pay the estimated tax at all if so desired. However, it is possible that not paying estimated tax could result in you being charged underpayment penalties when you complete and file your next return.
I too am self employed. Since my business income varies widely each and every year, I never pay what is recommended on the 1040-ES printouts. In fact, I don't use those forms at all. I just send the IRS 20% of my gross business earnings each quarter and call it good. I pay each quarter on line at www.irs.gov/payments. Then print the receipt and file it with my business records. That way, I have it at tax filing time.
For the 17 years I've been self-employed and using the 20% method I've never had an underpayment penalty. Usually I end up getting some of that money refunded. On three times in 17 years have I actually owed the IRS more, and all three times it was less than $1000. Still, no underpayment penalty was charged or assessed.
