| I think the primary concern is to avoid paying penalties and interest on the underpayment of your additional tax liability.
Ok, the IRS has made this fairly easy for taxpayers who are Non-Farmers, so long as they at least 110% of prior year tax liability, then they would not impose penalty on the underpayment of the tax liability.
Of course, you will have to pay the full balance of the tax due by April 15th with the filing of your tax return or with the extension of your tax return.
The other method to avoid penalty is to pay at least 90% of 2007 tax liability.
I would prefer the former method, as it is easily computable. | 
