Wow! In spite the little I know of our system I can realize the big differences between yours and ours.When I worked as a consultant, I got in the habit of providing my information to the accountant quarterly so I could make my estimated payments. The rule on how much you had to pay and when were arcane and difficult to follow. Since I have retired I have maintained that practice except for a different reason. When I take a mandatory withdrawals from my retirement account I can specify almost any amount to be withheld for taxes in the year I take that withdrawal. When you pay your taxes via "withholding" there are no penalties or interest that are assessed if you underpay what what you owe. If I engage in stock trading in December and have excessive capital gains. I just pay what I owe by April 15th with no muss and no fuss. This is in start contrast to the situation where unexpected payments from customers would come in in December. To avoid this problem in the past, because there was no withholding option, I would stop sending Invoices about Nov 15th to push the income into the following year.