@JaySF
If your income was not "earned evenly" throughout the year, and especially if you had a larger amount of income in the last quarter, is why people prepare the "annualized installment method" on their 2210 with their return...it is more work and you have to break down the income, withholding and deductions through the different periods.
If you break down your income, withholding and deductions per month, then you can add together whatever months are being included in each "period" for the 2210 Annualized method.
Also, here is a link to the Instructions for the 2210
IRS Instructions for Form 2210
and here is a link to the IRS Form 2210:
IRS Form 2210
