In order to avoid an estimated tax penalty, you must pay in at least 90% of the current years (2010) tax with your quarterly payments, or at least 100% of last years tax.
So, if you can look at your 2009 tax return, find the line that says TOTAL TAX. On form 1040, this will be line 60. Just make sure your total quarterly payments are more than this amount, and then you can pay the balance due from your sale on April 15, 2011. It's always nice to keep the cash in your bank for as long as possible.
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