On April 15, many self-employeddoctors took a double tax hit. They hadto pay Uncle Sam income taxes owed for2003, plus they had to come up with thefirst quarterly installment on their estimatedtaxes for 2004. The basic rule isthat you must make estimated tax paymentsif you have income that isn't subjectto withholding and the total tax onyour 2004 tax return will be more than$1000. One way to figure out how muchyou have to pay is to calculate the taxyou'll owe come next April and pay atleast 90% of that amount in quarterlyinstallments. If you guess wrong, though,you could be subject to penalties. A betterway for most taxpayers is to pay 100%of the tax owed on your 2003 tax return.Caveat: If your adjusted gross income isover $150,000, you must pay 110% ofthe amount of tax you owed for 2003.