As an addendum, if you overpay with your estimated tax payments, any over payment will be refunded to you at tax filing time. I've been self-employed for 20 years and I intentionally always overpay my estimated taxes - or at least a try to. That practically guarantees me a refund of the overpaid amount each year. Though I have had 2-3 years where I ended up owing a bit more than what I paid in quarterly taxes, I was still well within underpay limits so I didn't incur an underpayment penalty.
