the default method for penalties is that 1/4 of your taxes need to be paid in each quarter. withholding is allocated 1/4 to each quarter but estimates are allocated to the period paid. the only way to avoid penalties is to use the Annual Installment method for form 2210 or to use option A or B in Part II on the 2210 and provide a reason. the A/B method is at the discretion of the iRS.
