VolvoGirl gave you your answer:
Generally, most taxpayers will avoid this penalty if they owe less than $1,000 in tax after subtracting their withholdings and credits, or if they paid at least 90% of the tax for the current year, or 100% of the tax shown on the return for the prior year, whichever is smaller.
So it does not matter if you suddenly stopped making estimated tax payments, so long as you owe less at the end of the year than $1,000 or have paid at least 90% of the current year tax or 100% of the previous year's tax.
There is no penalty for stopping paying estimated taxes, but only for underpaying your taxes through the year.
If this still does not make sense, please see Estimated Taxes at the IRS website.
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