Right now it would be a good idea if you put aside aound 40%. it should not be that much , but if you take the amounts you have made in the first quarter of 2008, Add up the income,
if your married, deduct 10,900 Standard Deduction
Single 5,450
and
the dependency of $3500 each person in your family
from your net income for the quarter, including the Per Diem for the logged or proved days out, and multply by .153 % you will come up with your Social Security Tax owed on self employment.
then take the profit less the items above to come up with the tax owed anf figure if under 50000 you tax would be 15%, above that would be 25% to !00,000
then take the net income from the business, minus the standard deduction and the dependency deduction will give you the taxable income and figure it from there
Frank